Set a sensible g:sclandTerm when on Linux

This should allow Linux users to use their default terminal emulator,
rather than having to explicitly set one. Or at least it will on Debian
and Ubuntu based systems. I'm not sure what other systems use the
x-terminal-emulator alias.

I've also removed some trailing whitespace.
This is required on Linux (and other systems, I guess), as most terminals don't detach like gnome-terminal and OSX terminal does.

Can you test to see if this line has changed OSX performance? I believe it shouldn't, but I don't have an OSX machine to check with.