fixing using 3rd party mount tools from fstab

With the new mount system using fstab entries with 3rd party mount tools 
does not work any more, because mount -t only uses exec_mountprog for known 
mount types.

I have submitted a small patch that falls back to exec_mountprog if mount_fs 
fails. This way 3rd party mount tools will work again, without replacing 
base system mount tools.
http://www.freebsd.org/cgi/query-pr.cgi?pr=120784

I could have removed the use_mountprog(vfstype) function, but I left it in, 
because it documents which mount types in the base system still use the old 
style mount.