ProfileManager: Copy certs into individual files

Some users have reported problems storing certs in the SharedPreferences
(which apparently have a 8192-character limit per entry), so we'll copy
them into /data/data/app.openconnect/files instead.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
AndroidManifest: Bump version to 0.97-beta1 (971)

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
ProfileManager: Store relative paths to certs

This might help in cases where the configuration is backed up, then
subsequently restored to a different path (possibly on multiuser tablets).

Also, increase the file limit to 64kB, and make sure the output file is
executable, in case it's a CSD script.

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