scd: New option --debug-assuan-log-cats.

* scd/scdaemon.c (oDebugAssuanLogCats): New.
(opts): Add option --debug-assuan-log-cats.
(main): Implement option.
* common/asshelp.c (set_libassuan_log_cats): New.

The old way of setting the logging categories with an environment
variable is awkward if sdaemon is spawned from a running gpg-agent.