dirmngr: Fix regression in KS_GET for mail address pattern.

* dirmngr/ks-engine-hkp.c (ks_hkp_search): Munge mail address pattern.
(ks_hkp_get): Allow for mail addresses.
-

Before the keyserver changes in 2.2.28 gpg passed dirmngr a pail
address as an exact pattern (e.g. "=foo@example.org"). Since 2.2.28
the mail address is detected gpg gpg and we see for example
"<foo@example.org>". This patch fixes this to turn a mail address
into an exact match again.

• GnuPG-bug-id: T5497
• Signed-off-by: Werner Koch <wk@gnupg.org>