Partially revert r314948

While it sounds like a good idea to extract the RFC1048 data from PXE, in the
end it is not and it is causing lots of issues. Our pxeloader might need
options which are incompatible with other pxe servers (for example iPXE, but
not only).

Our pxe loaders are also now settings their own user class, so it is useful to
issue our own pxe request at startup

Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D10953