Btrfs-progs: btrfs-receive optionally honors the end-cmd

Btrfs-progs: btrfs-receive optionally honors the end-cmd

A new option is added to btrfs-receive to change the behavior when
an <end cmd> is received in the Btrfs send stream.

The traditional behavior (which still is the default) is to continue
to read the stream until an EOF condition is encountered. If an
<end cmd> is received, afterwards either an EOF or a new
<stream header> is expected.

The new behavior (if the -e option is set on the command line) is
to terminate after an <end cmd> is read without the need for an EOF.
This allows the stream (e.g. a single TCP stream) to carry additional
data or even multiple Btrfs send streams.

Old btrfs-send tools used to encode multiple snapshots like this
(with 2 snapshots in this example):
<stream header> + <sequence of commands> + <end cmd> +
<stream header> + <sequence of commands> + <end cmd> + EOF

If the new -e option is set, the expected format is like this:
<stream header> + <sequence of commands> +
                  <sequence of commands> + <end cmd>

The btrfs-send tool is changed in a seperate commit to always use
the new format, i.e. to send an <end cmd> only at the end.

Note that the currently existing receivers treat <end cmd> only as
an indication that a new <stream header> is following. This means,
you can just skip the sequence <end cmd> <stream header> without
loosing compatibility. As long as an EOF is following, the currently
existing receivers handle the new format (if the two new flags are
used) exactly as the old one.

The goal of changing the semantic of <end cmd> is to be able to use
a single stream (one TCP connection) to multiplex a request/response
handshake plus Btrfs send streams, all in the same stream. In this
case you cannot evaluate an EOF condition as an end of the Btrfs send
stream. You need something else, and the <end cmd> is just perfect
for this purpose.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>