edit: Make unit owner change away from POGO work

edit: Make unit owner change away from POGO work

Units owned by POGO are not in use.  Giving a unit to POGO destroys
it.  The opposite doesn't work, however: the unit prewrite hooks give
it right back to POGO, because efficiency is below the minimum.  Make
it work by also increasing efficiency to minimum.

Note that you can't use this to create a unit that doesn't already
exist in the respective file.  That's because edit's second argument
selects from existing objects only.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>