assuan: Reorganize waiting for socket.

assuan: Reorganize waiting for socket.

* common/asshelp.c (wait_for_sock): New function, collecting
codepaths from...
(start_new_gpg_agent) here and...
(start_new_dirmngr) here.
--

This has no functional change, but makes it easier to make this
function more efficient.

GnuPG-bug-id: 3490
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
(cherry picked from commit 0471ff9d3bf8d6b9a359f3c426d70d0935066907)

Gbp-Pq: Topic from-master
Gbp-Pq: Name assuan-Reorganize-waiting-for-socket.patch