renamed "args.toolchains" to "args.toolchain" because that looks better in the
help output:

```
./mpsd-software.py install --help
usage: mpsd-software.py install [-h] [--enable-build-cache] release toolchain [toolchain ...]
```

is better than

```
./mpsd-software.py install --help
usage: mpsd-software.py install [-h] [--enable-build-cache] release toolchains [toolchains ...]
```

because I wanted to know if package sets are included in the return value

- if passed to 'available', this creates a long and not very helpful message
  that lists all branches.
- if we catch it here, we avoid that.

amongst other things explain why 'available' is called 'available'