cli: make notmuch count --exclude a boolean argument

cli: make notmuch count --exclude a boolean argument

Commit 0f314c0c99be ("cli: convert notmuch_bool_t to stdbool")
over-eagerly converted EXCLUDE_TRUE and EXCLUDE_FALSE to EXCLUDE_true
and EXCLUDE_false in notmuch-count.c. We could just fix the case back,
but convert the option to an actual boolean argument instead.

We've used a keyword argument rather than a boolean argument for
notmuch count --exclude for five years, since commit 785c1e497f05
("cli: move count to the new --exclude=(true|false|flag) naming
scheme."), "to allow future options to be added more easily". I think
we can conclude future options aren't coming any time soon.