Stop hard-coding an -M flag to mdmfs(8) in /etc/rc.subr.
Now this flag can be set, or not set, for memory-backed
file systems on individual basis, as illustrated by the
rc.conf(5) variables tmpmfs_flags and varmfs_flags. The
flag is set for those FS'en by default, in /etc/defaults/rc.conf,
in order to stay compatible with the old rc.subr behaviour.

Submitted by: marck
MFC after: 3 days