xffm hangs on running external programs

[Jasper Huijsmans]

Hey edscott,

For me, xffm hangs every time it tries to run an external program. The
gui just freezes and only kill -9 can get rid of the process.

Apparently other people don't have this behaviour, but it is 100%
reproducibe for me.

Running it in gdb, shows the main thread exits normally when calling the
external program, leaving the other threads in a frozen state.

	Jasper