Make compression asymmetric by default and add warnings

Make compression asymmetric by default and add warnings

This commit introduces the allow-compression option that allow
changing the new default to the previous default or to a stricter
version.

Warning for comp-lzo/compress are not generated in the post option check
(options_postprocess_mutate) since these warnings should also be shown
on pushed options. Moving the showing the warning showing for
allow-compression to options_postprocess_mutate will complicate the
option handling without giving any other benefit.

Patch V2: fix spelling and grammer (thanks tincantech), also fix
   uncompressiable to incompressible in three other instances in the
   source code

Patch V3: fix overlong lines. Do not allow compression to be pushed

Patch V4: rename COMP_F_NO_ASYM to COMP_F_ALLOW_COMPRESS, fix style.
          The logic of warnings etc in options.c has not been changed
          since adding all the code to mutate_options would a lot more
          and more complicated code and after discussion we decided that
          it is okay as is.

Patch V5: Reword warnings, rebase on master

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20200626110554.3690-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20138.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>