Add more types to System.Posix.Types

A whole lot of types are still missing from System.Posix.Types as noted in a TODO comment in the code:

--- ToDo: blksize_t, clockid_t, blkcnt_t, fsblkcnt_t, fsfilcnt_t, id_t, key_t
--- suseconds_t, timer_t, useconds_t

Specifically the lack of a wrapper for blkcnt_t is causing trouble because it's forcing HFuse to re-implement unix's FileStatus type to include the missing st_blocks field. See FileStat docs on Hackage.

I have patches for base and unix ready to add wrappers for all the types the comment mentions, let's see if I can figure out how to use Phab ;)