MFC r192246:

do not create and mount new file systems on top of the old ones on every
invocation of this script once we already have one
(in case tmpmfs="YES").

MFC r221007:

Create a function for the code from r192246 so that it can be used both
times mount is called.

Limit the automatic behavior to when AUTO is specified (as it is in
etc/defaults/rc.conf) and for everything else take advantage of all
of the goodness in checkyesno.