Split tags: Push the maintainer view tag, where supported

Split tags: Push the maintainer view tag, where supported

We introduce a new `maint' possibility in to the dgit-tag-format
access cfg list, presence of which indicates that the receiver is
content to receive DEP-14-style maintainer-view (ie, non-dgit0 tags.

For now, we default this to true.  However, repos handled by
dgit-repos-server cannot currently cope with this, because:
 * dgit-repos-server currently tries to be compatible with old
   dgits which send DEP-14-named dgit tags;
 * dgit-repos-server doesn't currently permit the creation of
   `extraneous' (ie, non-dgit) tags and branches.

So for Debian, we do not set this flag.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>