bsdlabel: add deprecation notice

gpart is the preferred tool for managing partitions of all types,
including BSD disklabels.

Note that this is only about bsdlabel/disklabel, the tool -- there is no
current plan to remove support for MBR or BSD disk labels from the
kernel or from gpart.

Reviewed by: imp, olce
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43563

(cherry picked from commit b6f3cced151f8b7dc3e32977e8f1b826f0654690)