fuse.c: display pid/tid info on each access to FUSEG(active_object)

Test output with php examples/example_4.php /tmp/mountpoint -f and launch in another terminal
ls -la /tmp/mountpoint ; ls -la /tmp/mountpoint ; ls -la /tmp/mountpoint ; ls -la /tmp/mountpoint ; ls -la /tmp/mountpoint

Forcing fuse to single-thread mode with parameter -s "fixes" the problem.