add missing header
sync configure.ac with portable OpenSSH
sync with OpenSSH openbsd-compat/arc4random.c containing new ChaCha based random number generator for OpenBSD
while there, reduce delta between portable and master
sync with OpenSSH openbsd-compat/openbsd-compat.h
while there, cleanup and update openbsd-compat/NOTES
every portable .c file must have at the top of the file: