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.

Approved by: re (gjb)
Submitted by: freebsdnewbie_freenet.de
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27807

(cherry picked from commit e40787f900f3c262d5134d342e5a16757dd2193c)
(cherry picked from commit 1b2802ed4ed09746d6870080f85d79a9ccd782b0)