scd: Fix regression tracking the connection count.

* scd/scdaemon.c (get_active_connection_count): New.
(start_connection_thread): Bump ACTIVE_CONNECTIONS up and down.
* scd/command.c (cmd_getinfo): Add subcommand "connections".

Apply gpg-agent change to scdaemon. See the commit in 2016-08-06:

40d16029ed8b334c371fa7f24ac762d47302826e

Then, add kicking the loop, so that main loop can notice the change of
the connection.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>