Add 'linux_mounts_enable' rc.conf(5) variable, to make it possible
to disable mounting Linux-specific filesystems under /compat/linux
when 'linux_enable' is set to YES.

Reviewed by: netchild, ian (earlier version)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22320