* Add a way for a FUSE module to supply Haiku-specific extensions.
This allows it to integrate better with Haiku while only requiring
minimal changes on the FUSE module itself.
* For now, there is only one extension: another function pointer for
"get_fs_info", which lets the FUSE module fill in an fs_info struct.
FUSE provides no good way to otherwise communicate extra information,
such as the volume flags (e.g. B_FS_IS_SHARED).
* A FUSE module can signal that it supports the Haiku extensions by
a) defining HAS_HAIKU_FUSE_EXTENSIONS before including the fuse
headers
b) setting the global variable gHasHaikuFuseExtensions to 1 in
its initialization
Otherwise, the Haiku extensions are completely invisible to the
FUSE module.
