specs: specify timer math behaviours.

* specs/posix_spec.yaml (timeradd, timercmp, timersub): Specify
behaviours.
* lib/posix.lua.in (timeradd, timercmp, timersub): Argcheck
wrappers return inner function results.
(timercmp, timersub): Fix typo with initial definition scopes.

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