disallow non-absolute rpath $ORIGIN for suid/sgid/AT_SECURE processes

in theory non-absolute origins can only arise when either the main
program is invoked by running ldso as a command (inherently non-suid)
or when dlopen was called with a relative pathname containing at least
one slash. such usage would be inherently insecure in an suid program
anyway, so the old behavior here does not seem to have been insecure.
harden against it anyway.

ldso/dynlink.c

@@ -817,6 +817,9 @@ static int fixup_rpath(struct dso *p, char *buf, size_t buf_size)
 		origin = ".";
 		l = 1;
 	}
+	/* Disallow non-absolute origins for suid/sgid/AT_SECURE. */
+	if (libc.secure && *origin != '/')
+		return 0;
 	p->rpath = malloc(strlen(p->rpath_orig) + n*l + 1);
 	if (!p->rpath) return -1;