Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Sync with OpenBSD:

arc4random.c: In the incredibly unbelievable circumstance where
_rs_init() fails to allocate pages, don't call abort() because of
corefile data leakage concerns, but simply _exit(). The reasoning
is _rs_init() will only fail if someone finds a way to apply
specific pressure against this failure point, for the purpose of
leaking information into a core which they can read. We don't
need a corefile in this instance to debug that. So take this
"lever" away from whoever in the future wants to do that.

arc4random.3: reference random(4)

arc4random_uniform.c: include stdint.h over sys/types.h

Revert r346410 and r346411

libkern in .PATH has too many filename conflicts with libc and my -DNO_CLEAN
tinderbox didn't catch that ahead of time. Mea culpa.

Split arc4random_uniform into it's own file and sync with OpenBSD.

PR: 182610
Obtained from: OpenBSD
MFC after: 2 weeks