Don't use time_t in scfg_t definition

sbbsctrl.exe is built with Borland C++ still which only has a 32-bit time_t, so to keep the scfg_t definition compatible between MSVC and Borland builds, use time32_t (32-bit time_t) values only.

This fixes the reported sbbsctrl.exe error:
Error loading configuration
cfg->size (23944) != sizeof(scfg_t) (23952)

Introduced in commit e535aaac.
Reported via DOVE-Net by  Daryl Stout (TBOLT), confirmed by Nightfox (DIGDIST).