Split Chelsio send tags into a generic base tag and a ratelimit tag.

NIC KTLS will add a new TLS send tag type in cxgbe(4) that is a
distinct tag from a ratelimit tag. To support this, refactor
cxgbe_snd_tag to be a simple send tag with a type and convert the
existing ratelimit tag to a new cxgbe_rate_tag structure.

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D22072