common: Prepare for parsing mail sub-addresses.

* common/mbox-util.c (mailbox_from_userid): Add arg subaddress and
implement.  Change all callers to pass false for it.
* common/t-mbox-util.c (run_mbox_no_sub_test): New.
(run_filter): Add arg no_sub.
(main): Call new test and add option --no-sub.

Some stats: In the about 5300000 keys on the SKS servers we found 3055
unique mailboxes with a '+' in it. After removing leading and
trailing '+' as well as multiple '+' (e.g. "c++" or "foo+bar+baz")
2697 were left which seem to be valid sub-addresses.

To filter mailboxes out from a line delimited list with
user-ids (e.g. an SQL output), the command

t-mbox-util --verbose --filter

can be used; to output w/o sub-addresses add --no-sub.

• GnuPG-bug-id: T4200
• Signed-off-by: Werner Koch <wk@gnupg.org>