lib: add NOTMUCH_EXCLUDE_FLAG to notmuch_exclude_t

lib: add NOTMUCH_EXCLUDE_FLAG to notmuch_exclude_t

Add NOTMUCH_EXCLUDE_FLAG to notmuch_exclude_t so that it can
cover all four values of search --exclude in the cli.

Previously the way to avoid any message being marked excluded was to
pass in an empty list of excluded tags: since we now have an explicit
option we might as well honour it.

The enum is in a slightly strange order as the existing FALSE/TRUE
options correspond to the new
NOTMUCH_EXCLUDE_FLAG/NOTMUCH_EXCLUDE_TRUE options so this means we do
not need to bump the version number.

Indeed, an example of this is that the cli count and show still use
FALSE/TRUE and still work.