Added support for the Snappy compression algorithm

Added support for the Snappy compression algorithm which has shown to
have considerably better compression speed than LZO at a comparable
compression ratio.

To enable Snappy add:

  compress snappy

to both client and server config files.

Alternatively, enable compression framing on the client:

  compress

and have the server selectively push "compress snappy" to the client.

This change also extends the client capability handshake to include
IV_SNAPPY so the server can be aware that a connecting client supports
Snappy.

Note that the Snappy implementation also includes an improved framing
approach where the first byte of the compressed payload is replaced by
the compression control byte (the first payload byte is moved to the end
of the packet).  This solves off-by-one alignment issues, which improves
performance on ARM.

By default, the configure script will try to build with Snappy support.
To disable, use the --disable-snappy option.

The --enable-lzo-stub configure directive is now --enable-comp-stub
(because it's not actually "lzo" but "compression-enabled packet framing")

Add compression overhead to extra buffer unconditionally, as long
as USE_COMP is defined.

OpenVPN SVN r8206 (2.1.21a) and r8212 (2.1.21b)

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1366393268-27392-3-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7531
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Fix usage of 'compression ...' from global config.

The new compression framework introduced by SVN r8206 only works for
a P2MP server if the compression settings are re-installed for each
client via management interface, ccd/ file or client-connect script,
but the "global" settings get lost due to (needlessly) clearing the
c2.comp_context in the per-client instance.  Fixed.  (also trac#273)

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1366393268-27392-6-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7529