specl: luaposix >= v32 compatibility.

posix.timersub is now posix.sys.timersub.
* lib/specl/util.lua (timersub): Local handle for posix.timersub
with older luaposix releases, posix.sys.timersub for recent
releases otherwise nil for ancient releases.
(gettimeofday, timesince): Adjust.
* NEWS: Update.

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