* lib/timespec-add.c (timespec_add):
* lib/timespec-sub.c (timespec_sub):
Work even if time_t is narrower than int (a theoretical
possibility). Redo code for a bit more clarity.
* lib/timespec-sub.c (timespec_sub):
Fix off-by-2 bug if a.tv_sec == TYPE_MINIMUM (time_t) and 0 < b.tv_sec.
* modules/timespec-tests, tests/test-timespec.c: New files.
