Don't use boolean for losehp killer format type

Killer format isn't a boolean, since it has 3 possible values
(KILLED_BY_AN, KILLED_BY, NO_KILLER_PREFIX).  It shouldn't make any
difference behind the scenes, but it's confusing to use 'boolean' for
it.

src/hack.c

@@ -3645,7 +3645,7 @@ maybe_wail(void)
 }
 
 void
-losehp(int n, const char *knam, boolean k_format)
+losehp(int n, const char *knam, schar k_format)
 {
 #if 0   /* code below is prepared to handle negative 'loss' so don't add this
          * until we've verified that no callers intentionally rely on that */