crypto: Add crypto_cursor_segment() to fetch both base and length.

This function combines crypto_cursor_segbase() and
crypto_cursor_seglen() into a single function. This is mostly
beneficial in the unmapped mbuf case where back to back calls of these
two functions have to iterate over the sub-components of unmapped
mbufs twice.

Bump __FreeBSD_version for crypto drivers in ports.

Suggested by: markj
Reviewed by: markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30445