Set process title during zfs send.

This adds a '-V' option to 'zfs send', which sets the process title once a
second to the progress information.

This code has been in FreeNAS for a long time now; this is just upstreaming
it here. It was originially written by delphij.

Reviewed by: mav
Obtained from: iXsystems, Inc
Sponsored by: iXsystems, Inc
Differential Revision: https://reviews.freebsd.org/D19184