gpg,gpgsm: Move use-keyboxd to the new conf file common.conf

* common/comopt.c, common/comopt.h: New.
* common/Makefile.am: Add them.
* g10/gpg.c: Include comopt.h.
(main): Also parse common.conf.
* sm/gpgsm.c: Include comopt.h.
(main): Set a flag for the --no-logfile option.  Parse common.conf.
* tools/gpgconf-comp.c (known_options_gpg): Remove "use-keyboxd", add
pseudo option "use_keyboxd".
(known_pseudo_options_gpg): Add pseudo option "use_keyboxd".
(known_options_gpgsm): Remove "use-keyboxd".
* tests/openpgp/defs.scm (create-gpghome): Create common.conf.
* doc/examples/common.conf: New.

Note that --use-keybox still works but prints a warning. We will
eventually remove this option becuase it was marked as an experimental
feature anyway.

It would be too confusing if gpg and gpgsm use different key storages.
Further, other components (e.g. dirmngr or gpg-wks-client) which call
gpg or gpgsm need to be aware that the keyboxd is used and pass that
option on the command line. Now that common.conf is always read (even
if --no-options is used) those tools will work instantly.