subs/control: Grant bonus for capability security

subs/control: Grant bonus for capability security

Grant land units with security capability the same bonus as in convert
and shoot: multiply by (1 + eff/100).

military_control()'s code to count military is now functionally
equivalent to security_strength(), except it has no use for its second
parameter.  Change security_strength() to permit a null argument, and
reuse it in military_control().

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