agent: Change intialization of assuan socket system hooks.

* agent/gpg-agent.c (initialize_modules): Add hook again.
(main): Remove setting of the system houk but add scoket system hook
setting after assuan initialization.

Thread initialization is better to be deferred after fork (in case of
UNIX). assuan_sock_init should be earlier. Thus, we need to change
system hooks for assuan_sock_* interface. Or else, on Windows, it may
cause hang on server.

Updates-commit: 1524ba9656f0205d8c6ef504f773b832a7a12ab9

• GnuPG-bug-id: T3378
• Signed-off-by: Werner Koch <wk@gnupg.org>