Network fixes

Ports, DNS and other small fires. 8 commands.

• Find and stop what's using a portFor when you see "address already in use" or "port 3000 is already taken".
• Clear the DNS cacheWhen a site moved to a new server but your machine still goes to the old one.
• See which ports are open on your machineList every program listening for connections, with its port number.
• Check whether a port is reachableFind out if you can connect to a server on a given port, like a database or a website.
• Download a file from the terminalSave a file from a URL straight into the current folder.
• Find your IP addressYour local IP (on your home or office network) and your public IP (what websites see).
• Look up a domain's DNS recordsSee where a domain points, its mail servers and what a specific DNS server says.
• Point a domain at your own machine (hosts file)Make a name like myapp.test open your local app, by adding a line to the hosts file.