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.
Remove the `-lc++`, `-lc++abi`, and `-lunwind` options from the user
target, where they should be handled automatically. (ie, we're not using
`-nostdlib` or its bretheren.)
In order to allow -fpic and -fpie in the user target, move init to it's
own target -- it needs its own special build rules to make it loadable
by boot.
