the panic log

Add an optional paniclog file, controlled by a new PANICLOG macro that can
be used to log all panic and impossible messages.  Helpful when people
forget to send, or didn't see, the message.

include/extern.h

@@ -638,6 +638,7 @@ E void FDECL(check_recordfile, (const char *));
 #if defined(WIZARD)
 E void NDECL(read_wizkit);
 #endif
+E void FDECL(paniclog, (const char *, const char *));
 
 /* ### fountain.c ### */