[http] gPXE is a HTTP/1.0 client, not a HTTP/1.1 client

gPXE is not compliant with the HTTP/1.1 specification (RFC 2616),
since it lacks support for "Transfer-Encoding: chunked".  gPXE is,
however, compliant with the HTTP/1.0 specification (RFC 1945), which
does not require "Transfer-Encoding: chunked" to be supported.

The only HTTP/1.1 feature that gPXE uses is the "Host:" header, but
servers universally accept that one from HTTP/1.0 clients as an
optional extension (it is obligatory for HTTP/1.1).  gPXE does not,
for example, appear to support connection caching.  Advertising as a
HTTP/1.0 client will typically make the server close the connection
immediately upon sending the last data, which is actually beneficial
if we aren't going to keep the connection alive anyway.

src/net/tcp/http.c

 	if ( xfer_window ( &http->socket ) ) {
 		process_del ( &http->process );
 		if ( ( rc = xfer_printf ( &http->socket,
-					  "GET %s%s%s HTTP/1.1\r\n"
+					  "GET %s%s%s HTTP/1.0\r\n"
 					  "User-Agent: gPXE/" VERSION "\r\n"
 					  "Host: %s\r\n"
 					  "\r\n",