Rework shared memory to implement POSIX shm

[klange]

Our shared memory subsystem is similar enough to POSIX shm that it should be feasible to rework it to be POSIX compliant.

POSIX does not require shared memory objects to be interchangable with files, outside of using the file descriptor table to identify handles to them. As such, this work can be almost entirely independent of the new VFS.

Our shared memory subsystem is also the closest thing we have to an mmap, as it manages address space allocation, so it is a good place to begin work on a real mmap even if we're still not considering file mappings.

If we can migrate our exposed API to match the POSIX shm APIs, and make use of this in Yutani (for window textures and font caching, though with a real mmap the latter will become less relevant), it will bring us one step closer to our compositor and desktop application stack being portable to other OSes.

See also #410 which proposes similar plans for packetfs.

[klange]

Our current API compared to POSIX:

• shm_obtain is like a combination of shm_open and mmap.
• shm_release is a combination of munmap but also shm_unlink
• Internally, release_chunk is like shm_unlink except we're always tracking reference counts and and will "unlink" (removing the name mapping) at the same time we actually release resources, but these can be decoupled.