Stop using gcc linker magic

Somehow gcc figures out that nbd_err is err, because of the macro? not
sure.

This is confusing and black magic, stop doing it.

cliserv.c

@@ -72,7 +72,7 @@ void err_nonfatal(const char *s) {
 	fprintf(stderr, "Error: %s\n", s1);
 }
 
-void err(const char *s) {
+void nbd_err(const char *s) {
 	err_nonfatal(s);
 	fprintf(stderr, "Exiting.\n");
 	exit(EXIT_FAILURE);