Add SU connection type

[vpartington]

Overthere 2.3.1 added the ability to specify an alternative password to be entered on the intercepted password prompt when using the INTERACTIVE_SUDO connection type (#89). This makes it possible to use Overthere with the su command instead of the sudo command.

However, the user is still required to configure a number of other properties too (sudoCommandPrefix and quoteCommand).

Make the setup easier, and the support for su more obvious, by introducing a new SU connection type.