syslog: add LOG_MASK, and pass bits to openlog and setlogmask.

Close #156.
* specs/posix_spec.yaml (openlog): Move from here...
* specs/posix_deprecated_spec.yaml (openlog): ...to here.
* specs/posix_spec.yaml (setlogmask): Move from here...
* specs/posix_compat_spec.yaml (setlogmask): ...to here.
* specs/posix_spec.yaml (closelog, syslog): Move from here
* specs/posix_syslog_spec.yaml (closelog, syslog): New file.
...to here.
* specs/specs.mk (specl_SPECS): Add posix_syslog_spec.yaml.
* specs/posix_syslog_spec.yaml (openlog, setlogmask)
(LOG_MASK): Specify behaviour of new minimal bindings.
* ext/posix/compat.lua (openlog, setlogmask): Simplify
accordingly.
* lib/posix/deprecated.lua (openlog): Reimplement old string
specified option API in Lua over new minimal binding.
* ext/posix/syslog.c (PLOG_MASK): New function for converting
option priorities to mask bits.
* lib/posix/compat.lua (setlogmask): Reimplement old auto-ORing
API over new LOG_MASK and minimal setlogmask bindings.
* NEWS: Update.

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