convert shoot: Make security bonus proportional to efficiency

convert shoot: Make security bonus proportional to efficiency

Land units with capability security reduce the mobility cost and have
their military count double, regardless of efficiency.  This lets
players get the benefits of a security unit at a discount: just don't
build it beyond 10%.

Count security unit's military times 1 + eff/100 instead of double.
Change the mobility bonus term from number of security units to sum of
security unit efficiency / 100.  Partial fix for bug#64.

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