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