dhclient: Log a warning instead of bailing upon "illegal" options

In Azure, the DHCP servers add private option (id 0xf5), which contains
binary form of an IPv4 address. Once this option is converted to string
form, it could contain '$', e.g.

IPv4 address: 100.72.36.54
binary form: 0x64 0x48 0x24 0x36
string form: "dH$6"

dhclient bails upon "illegal" options like the above example, thus the
VM bring-up will fail.

Also as a side note, this "illegal" option detection was added in
OpenBSD ~11years ago:
http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sbin/dhclient/dhclient.c?rev=1.50&content-type=text/x-cvsweb-markup

And it was removed along with the removal of script support in OpenBSD
~3years ago:
http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sbin/dhclient/dhclient.c?rev=1.159&content-type=text/x-cvsweb-markup

Reported by: Hongxiong Xian <v-hoxian microsoft com>
Reviewed by: jhb, Dexuan Cui <decui microsoft com>
Tested by: Hongxiong Xian <v-hoxian microsoft com>
Analyzed by: Dong Liu <doliu microsoft com>
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5853