ktls: Add a routine to query information in a receive socket buffer.

In particular, ktls_pending_rx_info() determines which TLS record is
at the end of the current receive socket buffer (including
not-yet-decrypted data) along with how much data in that TLS record is
not yet present in the socket buffer.

This is useful for future changes to support NIC TLS receive offload
and enhancements to TOE TLS receive offload. Those use cases need a
way to synchronize a state machine on the NIC with the TLS record
boundaries in the TCP stream.

Reviewed by: gallatin, hselasky
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D32564