fusermount: close inherited fds

fusermount: close inherited fds

When using the auto_unmount option, the fusermount3
watchdog process retains any inherited file descriptors.
This PR ensures they are closed.

Reason is that FDs that are kept open for a long time might
cause issues for applications using libfuse, for example
if these expect a pipe close, but the pipe is kept open
through the inherited file descriptor.
See for example: https://github.com/cvmfs/cvmfs/issues/3645

Signed-off-by: MJ Harvey <mharvey@jumptrading.com>
Signed-off-by: Bernd Schubert <bschubert@ddn.com>