I noticed that programs running in the terminal had an open file
descriptor for the system theme buffer, inherited from the Terminal.
Let's be nice and always mark incoming fds with FD_CLOEXEC.
The PIDs were used for sharing shbufs between processes, but now that
we have migrated to file descriptor passing, we no longer need to know
the PID of the other side.
If you don't need a file descriptor after sending it to someone over
IPC, construct it with IPC::File(fd, IPC::File::CloseAfterSending)
and LibIPC will take care of it for you. :^)
When receiving a file descriptor over IPC, the receiver must now call
take_fd() on the IPC::File to take over the descriptor. Otherwise,
IPC::File will close the file on destruction.
