DHCPV4: add function ValidateOptions()

This add a new function ValidateOptions() in DHCPV4.
Codes for validating the DHCP options are removed and
unified into this new function.

Bug: 25642025
TEST=compile, unittest, and run it.
Change-Id: I92438469e6b1a0b1558a125dda2768db6f1092a1
2 files changed