OpenConnectManagementThread: Enforce a minimum MTU

Several users reported that they were running into bugs in Android
4.4.3/4.4.4 which caused VpnService.Builder.establish() to fail
catastrophically with low-ish (1200) MTUs.  So:

For IPv4 connections on <= 4.3.x, the new floor is 576.
For IPv6 connections anywhere, or for >= 4.4.x, the new floor is 1280.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Fix notification pending intents / back stack

Use the appropriate intent flags so that we don't wind up with multiple
copies of MainActivity on the task stack.  This applies to both the
"Configure VPN" and the "User input needed" pending intents.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Update changelog and bump version to 0.96 (969)

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>