gpg: Set a limit for a WKD import of 256 KiB.

* g10/call-dirmngr.c (MAX_WKD_RESULT_LENGTH): New.
(gpg_dirmngr_wkd_get): Use it.

WKD should return only a single key with just one UID. For key
rollover 2 keys may be send. A total of 256 KiB seems to be a
generous limit here.

• Signed-off-by: Werner Koch <wk@gnupg.org>