Revert back to mouting mfs vs tmpfs.
Revert back to mouting mfs vs tmpfs.

Apparently the tmpfs implementation has fallen behind in maintenance and
was therefore disabled by Theo de Raadt.

For more detail, see:
http://undeadly.org/cgi?action=article&sid=20160812011743

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

fnichol/openbsd-builder,fnichol/openbsd-builder