Clean up misuse of mpr() in shp_missile_defense()

Clean up misuse of mpr() in shp_missile_defense()

Don't use multiple calls of mpr() to print a single line, because that
creates a separate bulletin for each part.  The read command normally
merges the bulletins, but if the bulletins are more than five seconds
apart (clock jumped somehow), we get a bulletin header in the middle
of a line.

The mpr() misuse was introduced in Empire 2.