make dlopen honor the rpath of the main program

this seems to match what other systems do, and seems useful for
programs that have their libraries and plugins stored relative to the
executable.

src/ldso/dynlink.c

@@ -1220,7 +1220,7 @@ void *dlopen(const char *file, int mode)
 		p = 0;
 		errflag = 1;
 		goto end;
-	} else p = load_library(file, 0);
+	} else p = load_library(file, head);
 
 	if (!p) {
 		snprintf(errbuf, sizeof errbuf, noload ?