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
* make static all symbols which do not need to be exported
* rename main() to mount_FOO()
* new main() now just calls mount_FOO(), main() is only compiled in if
MOUNT_NOMAIN is not defined
* a_gid(), a_uid() and a_mask() were put into ../mount/fattr.[ch], local
versions removed