In particular, expose the system UUID as a setting ("smbios/uuid").

The PXE spec is (as usual) unclear on precisely when ProxyDHCPREQUESTs
should be issued.  We adapt the following, slightly paranoid approach:

  If an offer contains an IP address, then it is a normal DHCPOFFER.

  If an offer contains an option #60 "PXEClient", then it is a
  ProxyDHCPOFFER.  Note that the same packet can be both a normal
  DHCPOFFER and a ProxyDHCPOFFER.

  After receiving the normal DHCPACK, if we have received a
  ProxyDHCPOFFER, we unicast a ProxyDHCPREQUEST back to the ProxyDHCP
  server on port 4011.  If we time out waiting for a ProxyDHCPACK, we
  treat this as a non-fatal error.

Copy DHCP responses to a standard malloc()ed buffer, rather than
retaining the I/O buffer that they arrived in.

Allow for settings to be described by something other than a DHCP option
tag if desirable.  Currently used only for the MAC address setting.

Separate out fake DHCP packet creation code from dhcp.c to fakedhcp.c.

Remove notion of settings from dhcppkt.c.

Rationalise dhcp.c to use settings API only for final registration of the
DHCP options, rather than using {store,fetch}_setting throughout.

DHCP code was using an incorrect check for whether to construct a
DHCPDISCOVER or DHCPREQUEST packet.

Add dedicated functions create_dhcpdiscover(), create_dhcpack() and
create_proxydhcpack() for use by external code such as the PXE preboot
code.

Register ProxyDHCP options under the global scope "proxydhcp".

Unregister previously-acquired DHCP and ProxyDHCP settings when DHCP
succeeds.

Convert DHCP option applicators in dns.c and iscsi.c to settings
applicators.

Kill off DHCP option applicators.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>

RFC 4390 provides for the DHCP client identifier to contain the link-layer
hardware type and MAC address when the MAC address exceeds 16 bytes.
However, the hardware type field is only 8 bits; we were assuming 16 bits.

separate xfer method.

Add missing .alloc_iob entries to several xfer_interface_operations
structures.

their equivalent DHCP-option-specified values.

supply options.

also avoid waiting for ProxyDHCP offers).

Also reduce the ProxyDHCP timeout, because it's already irritating me.

to construct DHCP packets suitable for Infiniband.

to construct DHCP packets suitable for Infiniband.

(Problem observed by Clay McClure in VMware Fusion.)

order in which they occur.

to generate the initial transmission; this cuts off around 0.3s per
instantiated connection.

list.

Added FEATURE() macros to most relevant (non-driver) files.

and generate DHCP options to indicate features to DHCP server (and to
PXE NBPs).

xfer_window() function, which can return a scalar rather than a
boolean.

not apply DHCP options in dhcp_configure_netdev().

begin with.