Introduce channel flags in libcasper.

Instead of passing flags (which describe a type of nvlist)
every send/recv we remember them in channel.
It's enough for use to extract them only during unwrap.
This simplify use of Casper.

Reviewed by: bruffer@, bcr@ (both man page)
Differential Revision: https://reviews.freebsd.org/D14196 (man page)