Make dhclient(8) verify if new MTU (option 26) differs from current one and skip unneeded MTU change.
This check eliminates infinite loop of MTU change / link flap / lease verification / MTU change / link flap etc.
in case of some NIC drivers like em(4) or igb(4).

N.B.: obsolete u_int16_t is used in consistency with the rest of the file.

PR: 229432
Approved by: mav (mentor)
MFC after: 1 week