vrrpd: allow user to set priority = 255

vrrpd: allow user to set priority = 255

Too many problems with implicit ownership determination via duplicate
address assignment. Will revisit that in the future. For now, allow user
to specify 255 as a priority value. This is functionally no different
than any other priority value; it just serves as a self-documenting way
of saying you want one router to always be master.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>