fix readonly activation override options

fix readonly activation override options

This fixes a problem in commit e6bb780d242, in which the
back compat handling for the old locking_type=4 was
incorrectly translated to mean the same thing as --readonly,
which prevented activation because activation uses an
exclusive vg lock.  Previously, locking_type=4 allowed
activation.

If we see locking_type 4 in an old config, translate it to
the new combination of --readonly and --sysinit, which we
now define to mean the --readonly behavior with an exception
to allow activation.