refactor: maintain backwards compatibility for getpasswd.

Closes #148.
Separate backwards compatibility functions so that they still
work as before, but do not appear in the documentation.
* specs/posix_spec.yaml (creat, getgroup, getpasswd, getpid):
Move from here...
* specs/posix_compat_spec.yaml (creat, getgroup, getpasswd)
(getpid): New file. ...to here.
* lib/posix.lua.in (argerror, argtypeerror, checkstring)
(checktable, toomanyargerror, creat, getgroup, getpasswd)
(getpid): Move from here...
* lib/posix/compat.lua (argerror, argtypeerror, checkstring)
(checktable, toomanyargerror, creat, getgroup, getpasswd)
(getpid): New file. ...to here.
* NEWS: Update.

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