net: parse DHCP options from overloaded file/sname fields

If Option 52 in the vendor option field signals overloading
of the file and/or sname fields, these field may contain
additional options. Formatting of file/sname contained options
is the same as in the vendor options field, but without the
leading magic.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>