agent: Add experimental option --browser-socket.

* agent/agent.h (opt): Add field "browser_socket".
* agent/command.c (cmd_setkeydesc): Use a different message for
restricted==2.
* agent/gpg-agent.c (oBrowserSocket): New.
(opts): Add --browser-socket.
(socket_name_browser, redir_socket_name_browser): New.
(socket_nonce_browser): New.
(cleanup): Cleanup browser socket.
(main): Implement option.
(start_connection_thread_browser): New.
(handle_connections): Add arg listen_fd_browser and use it.

This is very similar to --extra-socket but intended to be used by a web
browser session. AS of now it only displays a different "Note: in
the Pinentry than --extra-socket but it may eventually be tweaked for
the use by browser extensions making use of gpg-agent.

It is marked experimental and and thus may be removed in later
versions.

To better support the different "client classes", it would be useful
to add corresponsing cache classes so that each class has its own
cache.

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