march: Fix 'm' not to leave engineer without mobility behind

march: Fix 'm' not to leave engineer without mobility behind

When lnd_sweep() rejects an engineer for want of mobility, it removes
it from the list of units.

Can happen only when sweeping for march sub-command 'm'.  Any engineer
without mobility is dropped from the march immediately.  Broken in
Empire 2.

Fix lnd_sweep() to handle this case just like the others, and like
shp_sweep(): report and continue with the next list member.

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