Add a new sesutil(8) utility

This is an utility for managing SCSI Enclosure Services (SES) device.

For now only one command is supported "locate" which will change the test of the
external LED associated to a given disk.

Usage if the following:
sesutil locate disk [on|off]

Disk can be a device name: "da12" or a special keyword: "all".

Reviewed by: mav
MFC after: 1 month
Relnotes: yes
Sponsored by: gandi.net
Differential Revision: https://reviews.freebsd.org/D3544