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.

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