net: dhcp: Allow "MAY_FAIL" to still try each adapter

This change allows the "MAY_FAIL" DHCP option to still attempt to
contact a DHCP server on each adapter and only give up once each
adapter has failed once.  To get the existing behavior, set the
already-existing ethrotate=no variable.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Cc: Keng Soon Cheah <keng.soon.cheah@ni.com>
Cc: Chen Yee Chew <chen.yee.chew@ni.com>