[build] Move headers out of target dirs

The great header shift: It didn't make sense to regenerate headers for
the same module for every target (boot/kernel/user) it appeared in. And
now that core headers are out of src/include, this was going to cause
problems for the new libc changes I've been working on. So I went back
to re-design how module headers work.

Pre-requisites:
- A module's public headers should all be available in one location, not
  tied to target.
- No accidental includes. Another module should not be able to include
  anything (creating an implicit dependency) from a module without
  declaring an explicit dependency.
- Exception to the previous: libc's headers should be available to all,
  at least for the freestanding headers.

New system:
- A new "public_headers" property of module declares all public headers
  that should be available to dependant modules
- All public headers (after possible processing) are installed relative
  to build/include/<module> with the same path as their source
- This also means no "include" dir in modules is necessary. If a header
  should be included as <j6/types.h> then its source should be
  src/libraries/j6/j6/types.h - this caused the most churn as all public
  header sources moved one directory up.
- The "includes" property of a module is local only to that module now,
  it does not create any implicit public interface

Other changes:
- The bonnibel concept of sources changed: instead of sources having
  actions, they themselves are an instance of a (sub)class of Source,
  which provides all the necessary information itself.
- Along with the above, rule names were standardized into <type>.<ext>,
  eg "compile.cpp" or "parse.cog"
- cog and cogflags variables moved from per-target scope to global scope
  in the build files.
- libc gained a more dynamic .module file

src/libraries/libc/signal.h

@@ -0,0 +1,86 @@
+#pragma once
+/* Signal handling <string.h>
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include "j6libc/config.h"
+#include "j6libc/cpp.h"
+#include "j6libc/sig_atomic_t.h"
+
+CPP_CHECK_BEGIN
+
+/* Signals ------------------------------------------------------------------ */
+
+/* A word on signals, to the people using PDCLib in their OS projects.
+
+   The definitions of the C standard leave about everything that *could* be
+   useful to be "implementation defined". Without additional, non-standard
+   arrangements, it is not possible to turn them into a useful tool.
+
+   This example implementation chose to "not generate any of these signals,
+   except as a result of explicit calls to the raise function", which is
+   allowed by the standard but of course does nothing for the usefulness of
+   <signal.h>.
+
+   A useful signal handling would:
+   1) make signal() a system call that registers the signal handler with the OS
+   2) make raise() a system call triggering an OS signal to the running process
+   3) make provisions that further signals of the same type are blocked until
+      the signal handler returns (optional for SIGILL)
+*/
+
+/* These are the values used by Linux. */
+
+/* Abnormal termination / abort() */
+#define SIGABRT 6
+/* Arithmetic exception / division by zero / overflow */
+#define SIGFPE  8
+/* Illegal instruction */
+#define SIGILL  4
+/* Interactive attention signal */
+#define SIGINT  2
+/* Invalid memory access */
+#define SIGSEGV 11
+/* Termination request */
+#define SIGTERM 15
+
+typedef void (*sighandler_t)(int);
+
+/* The following should be defined to pointer values that could NEVER point to
+   a valid signal handler function. (They are used as special arguments to
+   signal().) Again, these are the values used by Linux.
+*/
+#define SIG_DFL (sighandler_t)0
+#define SIG_ERR (sighandler_t)-1
+#define SIG_IGN (sighandler_t)1
+
+/* Installs a signal handler "func" for the given signal.
+   A signal handler is a function that takes an integer as argument (the signal
+   number) and returns void.
+
+   Note that a signal handler can do very little else than:
+   1) assign a value to a static object of type "volatile sig_atomic_t",
+   2) call signal() with the value of sig equal to the signal received,
+   3) call _Exit(),
+   4) call abort().
+   Virtually everything else is undefind.
+
+   The signal() function returns the previous installed signal handler, which
+   at program start may be SIG_DFL or SIG_ILL. (This implementation uses
+   SIG_DFL for all handlers.) If the request cannot be honored, SIG_ERR is
+   returned and errno is set to an unspecified positive value.
+*/
+sighandler_t signal( int sig, sighandler_t func );
+
+/* Raises the given signal (executing the registered signal handler with the
+   given signal number as parameter).
+   This implementation does not prevent further signals of the same time from
+   occuring, but executes signal( sig, SIG_DFL ) before entering the signal
+   handler (i.e., a second signal before the signal handler re-registers itself
+   or SIG_IGN will end the program).
+   Returns zero if successful, nonzero otherwise. */
+int raise( int sig );
+
+CPP_CHECK_END