qapi: Prepare for rejecting underscore in command and member names

qapi: Prepare for rejecting underscore in command and member names

Command names and member names within a type should be all lower case
with words separated by a hyphen.  We also accept underscore.  Rework
check_name_lower() to optionally reject underscores, but don't use
that option, yet.

Update expected test output for the changed error message.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323094025.3569441-23-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>