install-sx.sh: add more parameters and don't care about their order

Implements:

* replaces the recent `--develop` with `--branch <branch>`;
* adds `--branch-keep` to do no pulling, including static dependencies;
* adds `--testnet`;
* changes prefix specification: `--prefix <prefix>` insead of hard-coded first argument;
* allows simpler expansion in the future.

Issues:

* Should use `git checkout -t`?
* Which docs have to be updated?
* Anything else?