Fix WSL check for WSL 2 and simplify nohup in open_command

WSL 2 changes the output of `uname -r`. For instance,

  WSL 1: 4.4.0-18980-Microsoft
  WSL 2: 4.19.67-microsoft-standard

Since WSL 2 lowercases the M, we can match for the rest of the string
which remains lowercase throughout both versions. Another option would
be to match for both upper- and lower-case Ms, like that:

  $(uname -r) = *[Mm]icrosoft*

Fixed use of nohup in open_command where it was only necessary for
xdg-open (and actually harmful for cmd.exe in WSL 2). The current logic
is simpler and more future-proof.