gpgconf: Add command --query-swdb.

* tools/gpgconf.c (aQuerySWDB): New.
(opts): Add --query-swdb.
(valid_swdb_name_p): New.
(query_swdb): New.
(main): Implement command --query-swdb.

Right now this command is not very useful because dimngr has not yet
been changed to create the swdb.lst. For manual tests the swdb.lst
file from the Net can be used with these additional lines:

.filedate 20161102T130337
.verified 20161102T150000