Move "catch load" to a new file

The "catch load" code is reasonably self-contained, and so this patch
moves it out of breakpoint.c and into a new file, break-catch-load.c.
One function from breakpoint.c, print_solib_event, now has to be
exposed, but this seems pretty reasonable.

gdb/breakpoint.h

@@ -1803,4 +1803,10 @@ extern void catch_exception_event (enum exception_event_kind ex_event,
 				   const char *regex, bool tempflag,
 				   int from_tty);
 
+/* A helper function that prints a shared library stopped event.
+   IS_CATCHPOINT is true if the event is due to a "catch load"
+   catchpoint, false otherwise.  */
+
+extern void print_solib_event (int is_catchpoint);
+
 #endif /* !defined (BREAKPOINT_H) */