Inlined fuse_mount_help() into fuse_lowlevel_help().

Inlined fuse_mount_help() into fuse_lowlevel_help().

Both the BSD and Linux implementation actually accept mostly the same
FUSE-specific mount options. Up to now, the BSD help function appended
the output of ``mount_fusefs --help``, but looking at
http://www.unix.com/man-page/freebsd/8/mount_fusefs/ this is likely more
confusing than helpful (since the user is not actually invoking
mount_fusefs directly, most of the options don't make sense).