timespec_sign doesn't work with unsigned time_t

[Eli Zaretskii]

While I can understand why timespec_sign will never return a negative
value on platforms whose time_t is unsigned, this is at least worth a
note in the commentary to that function.  Evidently, even experts such
as Paul Eggert trip on these corner cases, see

  
http://git.savannah.gnu.org/cgit/emacs.git/commit/src/msdos.c?h=emacs-25&id=ccdaf04cfdaa42fb85d456274405cce32ee9f5d4

Thanks.