Fix pin bomb not to report subs when there are none

Fix pin bomb not to report subs when there are none

pin_bomb() computed the number of foreign subs as number of ships less
number of foreign surface ships.  This counted own surface ships as
subs.

Change it to count foreign subs directly.  Closes #906040.

However, shipsatxy(), the function for counting foreign ships, also
lists them.  Add a parameter to suppress that, and change its callers.