Don't reopen tun if cipher changes

Don't reopen tun if cipher changes

When the pulled options change, OpenVPN will attempt to reopen the tun
device.  That might fail if the process has already dropper privileges,
and is not needed unless the tun MTU is changed.  This patch therefore
ignores the cipher value for the digest if a fixed tun-mtu is used.

Additionally, this patch changes the md_ctx_update() call to include the
trailing zero byte of each option, to make sure that parsing "foo,bar"
results in a different hash than "foobar".  (Sorry for not catching that
during the review...)

The unit tests are a bit lame, but it secretly serves as a way to lower
the bar for adding more buffer.c unit tests.

Trac: #761
Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1481838366-32335-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13579.html
Signed-off-by: David Sommerseth <davids@openvpn.net>