ef_verify: Require land unit milit == 0 with capability spy

ef_verify: Require land unit milit == 0 with capability spy

The rules for spy units make little sense unless spies carry no
military.  For instance, spies die when they take casualties in ground
combat.  There seems to be a tacit assumption that spies have no
military.  I don't trust the code to behave unless this assumption is
met.  Require it before an overly adventurous deity finds out the
painful way that spies with military don't work.

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