add missing stdint. include for uint32_t, etc.

ok beck@ jsing@
remove dead code. these imsgs are handled in pony.c.
various queue improvements:

- add a "close" hook to the backend API.
- improve the sync() pattern in queue_fs: only sync at commit
  time and not for every envelope creation
- various fixes to the experimental external queue API.
Include -DLIBRESSL_INTERNAL in regress makefiles.

ok miod@
Typo in previous assym.h dependency changes
pedantic avoidance of division by zero, likely not actually
possible to hit for real.
ok miod@ jsing@
Remove undocumented _des_crypt() interface and its companion header file,
which had never been installed, so it's unlikely something ever used this
in the last 15~20 years.

ok deraadt@ jsing@ beck@
Define SMALL_REGISTER_BANK on arm and vax, for it generates faster code for
these systems (vax being 30% faster!). (surprisingly, the prime candidate for
SMALL_REGISTER_BANK, SuperH, runs actually slower in that case)