Properly support macOS in rootssh

[jalopezg-git]

Explain what you would like to see improved and how.

From the forum: https://root-forum.cern.ch/t/rootssh-from-mac-os-with-homebrew-fails-with-netcat/55593.

Apparently, rootssh uses a couple of Linux-specific things that prevent it to work on macOS, namely:

• The script relies on the OpenBSD netcat (GNU variant does not support UNIX-domain sockets), which is usually available as nc on macOS.
  On GNU/Linux, nc is typically a symlink to netcat; thus, I would advise to replace all occurrences of netcat to nc in the script.
• It also relies on xdg-open, that complies to the freedesktop.org's XDG specification, and that is expected to be present on most (all?) GNU/Linux systems.
  However, open should be used instead in macOS.

ROOT version

6.28/04

Installation method

Not specified.

Operating system

MacOS Ventura 13.4

Additional context

No response

[linev]

Hi, can you test changes from PR #13218?

[linev]

Should be fixed by #13218