gpg: Print a warning if GKR has hijacked gpg-agent.

* g10/call-agent.c (check_hijacking): New.
(start_agent): Call it.
(membuf_data_cb, default_inq_cb): Move more to the top.

Note that GUIs may use the gpg status line

[GNUPG:] ERROR check_hijacking 33554509

to detect this and print an appropriate warning.