Fix misdirected nuclear detonation output
Output went to the owner of the nuke instead of the player.
Fortunately, they're the same in normal usage. They can differ only
when a deity drops a foreign nuke from a foreign plane.
The fix also cleans up a misuse of mpr() in kaboom(): it used multiple
calls to print a single line, which 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. Fortunately, that
could happen only when a deity drops a foreign nuke. Before commit
a269cdd7 (v4.3.23), it could also happen for missions.
Broken in Empire 2.