Make all DHCP option numbers public.

This allows tests that emulate DHCP clients to include options.
The values are specified by the RFC and cannot change, so there
is no downside to making them public.

Test: builds
Bug: 150644681
Change-Id: I939bfac5ec14175b74ce3713f5af1362de577839
Merged-In: I939bfac5ec14175b74ce3713f5af1362de577839
(cherry picked from commit f75cd476)