Set MTU for device

The code tries setting the interface 'up' and 'running'. Setting the MTU may be required to get it running.
Set to a more widely used value
Android: updated README.md for clarity and added a bit more info. A few
tweaks to comments in the build script were also made.
Android: Updated the README.md to add the link to the tun installer
Android: Added a few more changes to the README for clarity.
Merge branch 'master' of git://github.com/prurigro/cjdns
Fix various grammar mistakes and wording on README
Merge branch 'grammar' of https://github.com/kbillings/cjdns into crashey
Merge branch 'patch-1' of https://github.com/descention/cjdns into crashey