Use compat.linux.emul_path instead of hardcoded path in /etc/rc.d/linux

In /etc/rc.d/linux the mounting paths of procfs, sysfs and devfs
are hardcoded to "/compat/linux". Switching to the content of
compat.linux.emul_path sysctl would allow to switch linuxulator
to different place.

Submitted by: freebsdnewbie_freenet.de
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27807