fusermount, libfuse: send value as unsigned in "user_id=" and "group_id="

...options.  Uids/gids larger than 2147483647 would result in EINVAL when
mounting the filesystem.  This also needs a fix in the kernel.
libfuse: highlevel API: fix directory file handle passed to ioctl() method

Reported by Eric Biggers
Advertize the existence of some "configure" env vars.

Advertize the existence of env vars MOUNT_FUSE_PATH, UDEV_RULES_PATH
and INIT_D_PATH in the execution of ./configure.
Initilaize stat buffer passed to ->getattr() and ->fgetattr()
to zero in all cases.

Reported by Daniel Iwan.
libfuse: document deadlock avoidance for fuse_notify_inval_entry()

and fuse_notify_delete()

Reported by Han-Wen Nienhuys