Windows shells like cmd and powershell needs to use different quoting/escaping
of strings and paths than the unix shell, and also absolute paths must be fixed
by removing leading slash that the POSIX formatted paths have
(e.g. /C:/Users does not work in shell, it must be converted to C:/Users).
Tries to autodetect shell type (cmd, powershell, unix) on first use.
Implemented default builtin powershell functions for hashsum and about when remote
shell is powershell.
See #5763Fixes#5758
