Add the capability to refresh the gpart(8) label without need a reboot.

gpart(8) has functionality to change the label of an GPT partition.
This functionality works like it should, however, after a label change
the /dev/gpt/ entries remain unchanged. glabel(8) status output remains
unchanged. The change only takes effect after a reboot.

PR: 162690
Submitted by: sub.mesa@gmail, Ben RUBSON <ben.rubson@gmail.com>, ae
Reviewed by: allanjude, bapt, bcr
MFC after: 6 weeks.
Differential Revision: https://reviews.freebsd.org/D9935