Use this routine both in mount_fs and rump_fs to provide equivalent
command line parameters and therefore usage interchangeability.
While doing this, combine some common mountgoop to mountprog.h
that require all of /sbin/mount_* and /sbin/umount to be modified:
* incorporate vfslist.c, adding checkvfsname() and makevfslist()
* getmntopts() now takes 4 args (adding int *altflagp)