Use different readline buffers in Android and interactive Python shells

[ghost]

When entering an Android or interactive Python shell, readline is left enabled. This means that we still provide readline support, but the buffer is shared with the main Mercury application. This sucks, because we start providing Mercury commands in the Linux shell and vice-versa.

We also provide Mercury command-line completion where it is not appropriate.

It would be better if we could distingush between different environments, and swap out the completer with a more appropriate one.