Bugfix: mark IPC fd CLOEXEC (Thanks Layus)

Bugfix: mark IPC fd CLOEXEC (Thanks Layus)

Without this fix, children of i3bar would inherit the file descriptor of
the IPC connection to i3. Therefore, even if i3bar exits with SIGSEGV,
the connection to i3 stays open. Because nobody actually reads any
messages by i3, the buffer will fill up and i3 can’t deliver any more
messages, and thus busy-loops at that point.

fixes #995