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: T3490
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>