[ld.so] Add a minimal dynamic linker

This commit includes a number of changes to enable loading of PIE
executables:

- The loader in srv.init checks for a `PT_INTERP` segment in the program
  its loading, and if it exists, loads the specified interpreter and
  passes control to it instead of the program itself.
- Added ld.so the dynamic linker executable and set it as the
  interpreter for all user-target programs.
- Program initial stack changed again to now contain a number of
  possible tagged structures, including a new one for ld.so's arguments,
  and for passing handles tagged with protocol ids.
- Added a stub for a new VFS protocol. Unused so far, but srv.init will
  need to serve VFS requests from ld.so once I transition libraries to
  shared libs for user-target programs. (Right now all executables are
  PIE but statically linked, so they only need internal relocations.)
- Added 16 and 8 bit variants of `util::bitset`. This ended up not being
  used, but could be useful.

src/user/srv.init/init.module

@@ -7,6 +7,7 @@ init = module("srv.init",
     ld_script = "init.ld",
     sources = [
         "acpi.cpp",
+        "initfs.cpp",
         "j6romfs.cpp",
         "loader.cpp",
         "main.cpp",