easier basic commands

[totaam]

The most common command that I use is:

xpra start --start=xterm

There is no reason why we can't make it easier and support:

xpra start xterm

[totaam]

The commit above does this and is quite small too.

We also automatically enable:

• --exit-with-children since the command(s) are added as start-child options
• --attach is enabled if we have a real display (ie: MacOS, Windows or others as long as it's not an SSH session)

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This allows us to do things like:

xpra desktop fluxbox

To start fluxbox in a desktop session and terminate the session when fluxbox exits.

It also supports remote hosts:

xpra start ssh://user:password@host:sshport/ xterm firefox