Fixes #391

The proposed addition does not go into the mechanism (the names of the specific environment variables or the PowerShell parameters) but is more express about in what manner the behaviour of the installation scripts can be changed.

Introduces that important flexibility as the first topic under 'More on installation'.

Explains that the PowerShell script finally (by default) runs the script for Unix-like operating systems (so a Windows user understands better that the environment variables in the former are applicable to both, and what is meant by the 'final' bootstrap script in the content of the Windows parameters).

Refers to what has gone before, under 'Continuous integration', rather than repeat the added content.

This is a major refactor of some CLI code. We try to distinguish
GHC versions from other versions, so that we can use distinct parsers.

Hopefully this doesn't introduce new bugs.

This also forces ghcup run to use the new internal ~/.ghcup/tmp dir.