Avoid double unmount on FUSE_DESTROY

This is a long standing issue, a system could have unmounted
/path/to/mnt and then fuse-client/kernel would send FUSE_DESTROY,
which would then again try a umount.

Given that FUSE_DESTROY is async, that umount might arrive
any time later and might possibly unmount a wrong mount point.
A warning as in issue #1286 is just minor to that.

Code wise this uses atomics to free the char *, as FUSE_DESTROY
might race with a signal and a double free might be possible
without proctection. A lock might run into the same issue,
if the signal would arrive at the wrong time a double lock
would be possible.

Additionally, fuse_session_mount() is updated, to first
duplicatate the pointer and to then do the kernel mount -
reverting the kernel mount in case of strdup() failure
is much harder.

Closes: https://github.com/libfuse/libfuse/issues/1286
Signed-off-by: Bernd Schubert <bernd@bsbernd.com>

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