Refactor paths handling / Support backup/remove executables from different environments

[desseim]

The path parameter preparation / sanitation at the beginning of the script could do with some refactoring, to:

1. extract path calculation to functions
2. store Windows / WSL path sets separately
3. automatically use the path set which matches the environment of the called executable

In other terms, something like this crude pseudocode:

function CalculateWinPath {
    ...
    return $WinPathVars
}
function CalculateWSLPath {
    ...
    return $WSLPathVars
}

# before rdiff-backup execution
$Paths[[RdiffBackupExeEnv]::Win] = CalculateWinPath
$Paths[[RdiffBackupExeEnv]::WSL] = CalculateWSLPath

# when referring to paths:
$Paths[$RdiffBackupExe.Environment]  # for example

Currently it only calculates, stores and uses one type of paths, depending on the environment of the rdiff-backup executable set to be called. This requires both "backup" and "remove" executables to run in the same environment (i.e. Windows or WSL).
This refactoring would allow for removing this current restriction.