Drop the use of nohup on OSX for tmux compatibility

[mcornella]

Apparently nohup is not needed in OSX, and using it has the side that it may break under tmux with the error

nohup: can't detach from console: No such file or directory

For now, this commit only drops the use of nohup in OSX. But it may not be necessary under other similar platforms.

Fixes #4520

[apjanke]

Works more or less as expected for me on OS X 10.9.5, in and out of tmux. (That is, the file gets opened in native app per file associations, and command returns immediately.)

However, it always returns true, even when given a bad argument. I think the underlying commands all report a useful exit status; it would be nice to expose that.

[~]
$ open no-such-file.txt
The file /Users/janke/no-such-file.txt does not exist.
[✘ ~]
$ echo $?
1
[~]
$ open_command no-such-file.txt
[~]
$ echo $?
0

I don't think backgrounding or disowning the job is necessary on OS X; open uses IPC to invoke an application that's in the desktop session and is a child of launchd, and hands the file off to it. Similar with cygstart on Windows; the Windows program that it hands off to is running in the desktop session, and isn't a child of the shell or terminal. Since all these "open" commands are IPC handoff things, backgrounding or disowning might not be needed at all. And in the case where there was an error doing the handoff or resolving a program, I think you'd want that status info synchronously.

[mcornella]

Ok I've replaced the current solution with just droping nohup under OSX. I can't test that this works everywhere on OSX but you seem pretty confident that open doesn't need to be disowned. In fact, xdg-open in my debian system also behaves the same way, but I recall some system where calling that without nohup didn't work well.

Anyway, I'm finally merging this one.