[params] Add support for the general concept of a form parameter list

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[cmdline] Add "params" and "param" commands to manage form parameter lists

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[uri] Allow URIs to incorporate a parameter list

HTTP POST requires the ability to associate a parameter list with a
URI.  There is no standardised syntax for this.  Use a non-standard
URI syntax to incorporate the specification of a parameter list within
a URI:

  URI = [ absoluteURI | relativeURI ]
	[ "#" fragment ] [ "##params" [ "=" paramsName ] ]

e.g.

  http://boot.ipxe.org/demo/boot.php##params
  http://boot.ipxe.org/demo/boot.php##params=mylist

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[http] Add support for HTTP POST

Allow HTTP POST requests to be generated when the URI includes a
parameter list.  For example:

  #!ipxe
  params
  param mac ${net0/mac}
  param uuid ${uuid}
  param asset ${asset}
  chain http://boot.ipxe.org/demo/boot.php##params

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