Identify the FD hold by parent process

Identify the FD hold by parent process

Issue:
There is no directly way to get the FD
hold by parent process which asked do fuse mount.

Use Case:
For auto_unmount case, identify the FD can easy to close
the FD and make automatically unmount manually and explicitly.
The FD[1] can be got via getenv(FUSE_COMMFD2_ENV).
One potential use case is to satisfy FD-Leak checks.

Solution:
Add an extra env _FUSE_COMMFD2 to store the FD.
This will provide a easy way to get the FD via FUSE_COMMFD2_ENV.

Signed-off-by: Zegang Luo <zegang.luo@qq.com>