Pass a "FREEBSD" user-class in PXE dhcp request

rfc3004 allows to pass multiple user classes on dhcp requests
this is used by dhcp servers to differentiate the caller if needed.

As an example with isc dhcp server it will be possible to make options
only for the FreeBSD loaders:

if exists user-class and option user-class = "FREEBSD" {

option root-path "tftp://192.168.42.1/FreeBSD;

}

Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D10951