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

[jhb]

Summary

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.

Suggested by: markj

[jhb]

I originally had this function return both values by reference, but lots of places use different pointer types (e.g. char * and const char *) and I didn't want to add a lot of casts to deal with that, so I changed it to return the pointer. I did have to change a few variables to size_t in the later changes to cryptosoft to cope with the length being set by reference instead of returned, but that was less disruptive overall.

[bcr]

Ok from manpages.

[markj]

Thank you.