sync with OpenSSH openbsd-compat/setproctitle.c
while there, add openbsd-compat/bsd-misc.{c,h}, and update openbsd-compat/openbsd-compat.h
bye bye arc4random(). Chacha is used everywhere:
- no need to check for arc4random_*() in configure
- use crypto safe PRNG csprng_*() wrapper around chacha_*()
- use csprng_*() everywhere in regress/ and smtpd/ (including backends and filters)
- contrib/libexec/encrypt/encrypt.c and openbsd-compat/mktemp.c still use chacha_*()
do not substitute paths in manpages and configuration files, to let packagers put their wished paths.

fixes #326