Add stateless RPC options to upload-pack, receive-pack

Add stateless RPC options to upload-pack, receive-pack

When --stateless-rpc is passed as a command line parameter to
upload-pack or receive-pack the programs now assume they may
perform only a single read-write cycle with stdin and stdout.
This fits with the HTTP POST request processing model where a
program may read the request, write a response, and must exit.

When --advertise-refs is passed as a command line parameter only
the initial ref advertisement is output, and the program exits
immediately.  This fits with the HTTP GET request model, where
no request content is received but a response must be produced.

HTTP headers and/or environment are not processed here, but
instead are assumed to be handled by the program invoking
either service backend.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>