sys: Remove $FreeBSD$: two-line .h pattern

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

libkern: Add arc4random_uniform

This variant get a random number up to the limit passed as the argument.
This is simply a copy of the libc version.

Sponsored-by: The FreeBSD Foundation
Reviewed by: cem, hselasky (previous version)
Differential Revision: https://reviews.freebsd.org/D24962

libkern: Bring in arc4random_uniform(9) from libc

It is a useful arc4random wrapper in the kernel for much the same reasons as
in userspace. Move the source to libkern (because kernel build is
restricted to sys/, but userspace can include any file it likes) and build
kernel and libc versions from the same source file.

Copy the documentation from arc4random_uniform(3) to the section 9 page.

While here, add missing arc4random_buf(9) symlink.

Sponsored by: Dell EMC Isilon