libfuse: set close-on-exec flag on pipe file descriptors

libfuse: set close-on-exec flag on pipe file descriptors

The FUSE library may be used from any number of programs which
may also fork() + execve(), so set the close-on-exec flag to
avoid inadvertant leakage of pipe file descriptors.

While we're at it, attempt to use pipe2() since this is within a
(currently) Linux-only code path and pipe2() offers thread-safety.