- Implement proper unprivilegied user permission verifications
Verification is now done in the lookup method, as it is the way to
go. Of course there are corner cases, such as the sticky bit which
need special handling in the remove method.

- Set full fsidx in vftstat method

- Do not pass O_APPEND to the filesystem. FUSE always sends the
write offset, so setting O_APPEND is useless. If the filesystem
uses it in an open(2) system call, it will even cause file
corruptions, since offsets given to pwrite(2) will be ignored.
This fix allows glusterfs to host a NetBSD ./build.sh -o build

- Do not use the FUSE access method, use getattr and check for
permission on our own. The problem is that a FUSE filesystem will
typically use the Linux-specific setfsuid() to perform access
control. If that is missing, any chack is likely to occur on
behalf of the user running the filesystem (typically root), causing
access method to return wrong information.

- When possible, avoid performing a getattr method call and use
cached value in puffs_node instead. We still retreive the latest
value by calling getattr when performing append write operation,
to minimize the chances that another writer appended since the
last time we did.

- Update puffs_node cached file size in write method

- Remove unused argument to perfuse_destroy_pn()

(manu)