bugfix: preserve effective uid and gid for new files

[rr-]

Closes #3

Inspired by https://github.com/mpartel/bindfs/blob/master/src/bindfs.c#L425-L472

[rr-]

Of course, this solution will fail to lchown anything if the mount user isn't root, but same happens with sshfs:

user@localhost:~$ sshfs -o allow_other example.com: ~/test/
user@localhost:~$ touch ~/test/a
user@localhost:~$ sudo touch ~/test/b
user@localhost:~$ sudo ls ~/test/{a,b}
-rw-r--r-- 1 user user      0 Jan  1  1970 a
-rw-r--r-- 1 user user      0 Jan  1  1970 b

What matters is when things happen the other way: the directory is mounted under root (for example through /etc/fstab) and an ordinary user tries to create a file within the mounted directory. This ensures the file will belong to the user rather than root.

This solution doesn't support idmaps.

[fbarriga]

thanks!