ask-password: check keyring in ask_password_tty and ask_password_agent

A race condition happens when calling ask_password_auto() multiple times
to unlock several disks on boot and effectively no password caching is
utilized. This patch fixes it by polling the cache when waiting for
the password.

src/test/test-ask-password-api.c

@@ -3,15 +3,17 @@
 #include "alloc-util.h"
 #include "ask-password-api.h"
 #include "log.h"
+#include "strv.h"
 
 static void ask_password(void) {
         int r;
-        _cleanup_free_ char *ret;
+        _cleanup_strv_free_ char **ret = NULL;
 
         r = ask_password_tty(-1, "hello?", "da key", 0, 0, NULL, &ret);
         assert(r >= 0);
+        assert(strv_length(ret) == 1);
 
-        log_info("Got %s", ret);
+        log_info("Got %s", *ret);
 }
 
 int main(int argc, char **argv) {