Add support for kgdb's 'detach' command.

Reviewed by:	marcel
Sponsored by:	Network Appliance

sys/gdb/gdb_main.c

@@ -145,6 +145,11 @@ gdb_trap(int type, int code)
 			gdb_listening = 1;
 			return (1);
 		}
+		case 'D': {     /* Detach */
+			gdb_tx_ok();
+			kdb_cpu_clear_singlestep();
+			return (1);
+		}
 		case 'g': {	/* Read registers. */
 			size_t r;
 			gdb_tx_begin(0);