agent: separate out daemon handling infrastructure for reuse

* agent/call-scd.c: Factor re-usable code out to ...
* agent/call-daemon.c: new.  Store infos in an array to allow for
other backend daemons.
* agent/Makefile.am (gpg_agent_SOURCES): Add new file.
* agent/agent.h: Include assuan.h.
(enum daemon_type): New.
(opt): Replace scdaemon_program by daemon_program array.  Replace
scd_local by a array d_local.  Change users accordingly.

The model I'm using for a TPM daemon is the current scdaemon. That
includes start and stop handlers plus liveness checks and an assuan
socket generator. To avoid massive code duplication (and save me a
lot of effort), I've elected to strip this code out of call-scd.c into
a generic framework which can then be reused as is by the TPM handling
daemon.

Modified original patch for 2.2 heavily to fit the new framework used
in master (gnupg 2.3)

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