Add Posix_Free and try harder to avoid clearing errno

Add Posix_Free and try harder to avoid clearing errno

free() can clear errno.  Apparently POSIX will prohibit that
in a future version of the standard; in the meantime, add a
Posix_Free function that guarantees errno preservation and
start using it.

Similarly, make Util_ZeroFree, Util_FreeList, etc. also
preserve errno.