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.

doc/gpg.texi

@@ -2476,7 +2476,8 @@ opposite meaning. The options are:
   signatures.  Defaults to yes.
 
   @item bulk-import
-  When used with --use-keyboxd do the import within a single
+  When used the keyboxd (option "use-keyboxd" in @file{common.conf})
+  do the import within a single
   transaction.  This is an experimental feature.
 
   @item import-minimal
@@ -3753,6 +3754,12 @@ current home directory (@pxref{option --homedir}).
   name may be changed on the command line (@pxref{gpg-option --options}).
   You should backup this file.
 
+  @item common.conf
+  @efindex common.conf
+  This is an optional configuration file read by @command{@gpgname} on
+  startup.  It may contain options pertaining to all components of
+  GnuPG.  Its current main use is for the "use-keyboxd" option.
+
 @end table
 
 Note that on larger installations, it is useful to put predefined files