qapi: Document flaws in checking of names

We don't actually enforce our "other than downstream extensions [...],
all names should begin with a letter" rule.  Add a FIXME.

We should reject names that differ only in '_' vs. '.'  vs. '-',
because they're liable to clash in generated C.  Add a FIXME.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>