Graphene emulates `/proc/[tid|pid]` subdirectory using the pseudo-FS
logic under fs/proc/thread.c and fs/proc/ipc-thread.c. This commit
refactors these files: name changes, better structure, using string.h
functions, small bug fixes. The IPC-thread logic is significantly
simplified and partly commented out (it was never used and probably
will never be used, but still keeping it just in case). The LibOS
`proc_common` test is augmented to test more functionality.