(Ab)use the "secs" field in transmitted DHCP packets to convey
metadata about the DHCP session state.  In particular:

  bit 0 represents the receipt of a ProxyDHCPOFFER
  bit 1 represents the receipt of a DHCPOFFER
  bits 2+ represent the transmitted packet sequence number

This allows some relevant information about the internal state of the
DHCP session to be read out from a packet trace from a non-debug build
of iPXE.  It also potentially allows replies to be correlated to their
requests (for servers that copy the "secs" field from request to
reply).

Signed-off-by: Michael Brown <mcb30@ipxe.org>