Split kern_break from sys_break and use it in linuxulator

Previously the linuxulator's linux_brk invoked the FreeBSD sys_break
syscall implementation directly. Instead, move the bulk of the existing
implementation to kern_break, and call that from both sys_break and
linux_brk.

This also addresses a minor bug in linux_brk in that we now return the
actual (rounded up) break address, rather than the requested value.

Reviewed by: brooks (earlier version)
Sponsored by: Turing Robotic Industries
Differential Revision: https://reviews.freebsd.org/D16019