sys.stat: wrap umask minimally.

* specs/posix_spec.yaml (umask): Move from here...
* specs/posix_compat_spec.yaml (umask): ...to here.
* specs/posix_sys_stat_spec.yaml (umask): Specify minimal binding
for umask(2).
* ext/posix/sys/stat.c (umask): Simplify drastically.
* ext/include/_helpers.c (pushmode): Remove.  No longer used.
* lib/posix/compat.lua (rwxrwxrwx, octal_mode, mode_munch):
Translated from C equivalents in ext/include/_helpers.c.
(umask): Use them to reimplement legacy modestring based API.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>