Make command exec functions return an enum rather than -1/0/1 values and

add a new value to mean "leave client running but don't attach" to fix
problems with using some commands in a command sequence. Most of the
work by Thomas Adam, problem reported by "jspenguin" on SF bug 3535531.

cmd.c

@@ -280,7 +280,7 @@ usage:
 	return (NULL);
 }
 
-int
+enum cmd_retval
 cmd_exec(struct cmd *cmd, struct cmd_ctx *ctx)
 {
 	return (cmd->entry->exec(cmd, ctx));