bectl: simplify the tail end of the jail cmd

This has already confused me once (and I'm pretty sure I wrote it), so let's
clarify: unjailing after the command has completed will only happen if we're
interactive and -U has not been specified.

This just folds two conditionals together to make it obvious how -b/-U
interact with each other.

MFC after:	3 days

sbin/bectl/bectl_jail.c

@@ -356,10 +356,8 @@ bectl_cmd_jail(int argc, char *argv[])
 	}
 	free(jargv);
 
-	if (!interactive)
-		return (0);
-
-	if (unjail) {
+	/* Non-interactive (-b) mode means the jail sticks around. */
+	if (interactive && unjail) {
 		/*
 		 *  We're not checking the jail id result here because in the
 		 *  case of invalid param, or last command in jail was an error