Adjust Database{Location} to only attempt to create a mimetype when
actually necessary, and fail otherwise if a writable version doesn't yet
exist. Correspondingly, adjust callers such as
DatabaseLocation::DeleteAttribute(). Fixes a problem where a caller asking
to perform a mimeset could fail early due to SetSupportedTypes() attempting
to update the read-only mime database entry supplied by a package, and
consequently most of the mimeset operations would be skipped.
701a5d6b79