return LIBAVRDUDE_EXIT instead of exit(0)

src/jtagmkII.c

@@ -1382,7 +1382,7 @@ static int jtagmkII_parseextparms(const PROGRAMMER *pgm, const LISTID extparms)
       if (pgm->flag & PGM_FL_IS_PDI)
         msg_error("  -xrtsdtr=low,high       Force RTS/DTR lines low or high state during programming\n");
       msg_error(  "  -xhelp                  Show this help menu and exit\n");
-      exit(0);
+      return LIBAVRDUDE_EXIT;;
     }
 
     pmsg_error("invalid extended parameter '%s'\n", extended_param);

src/libavrdude.h

@@ -59,6 +59,7 @@ typedef uint32_t pinmask_t;
 #define LIBAVRDUDE_NOTSUPPORTED (-2) // operation not supported
 #define LIBAVRDUDE_SOFTFAIL (-3) // returned, eg, by avr_signature() if caller
                                  // might proceed with chip erase
+#define LIBAVRDUDE_EXIT (-4)     // End all operations in this session
 
 /* formerly lists.h */