Remove support for unsigned time_t

Remove support for unsigned time_t

We've never actually tested this support, and we should probably assume
it's broken.

To the best of my knowledge, only OpenVMS has this, and even on
OpenVMS it's a compile-time option to disable it.  And I don't think
we build on openvms anyway.  (Everybody else seems to be working
around the 2038 problem by using a 64-bit time_t, which won't expire
for roughly 292 billion years.)

Closes ticket 18184.