userlandfs: compatibility with FUSE 2.9 and FUSE lowlevel API

- Import latest version of files from FUSE 2.9.9 (our last
synchronization was with 2.7.4)
- Adjust fuse pkgconfig file to use the POSIX error mapper
automatically, since that's required for all FUSE software
- Implement the lowlevel API in addition to the highlevel one. The
lowlevel API uses inode numbers to identify files, rather than paths,
making it a better fit to the userlandfs architecture.

The FUSE 2.x branch is not maintained anymore by FUSE developers,
however, pretty much no one migrated to FUSE 3.x. So it is more
interesting to implement, rather than 3.x.

Confirmed still working with sshfs and curlftpfs.

Example use:

I tested this with github.com/whoozle/android-file-transfer-linux

- Build the fuse library and copy it to ~/config/non-packaged/add-ons/userlandfs/
- Start the server: /system/servers/userlandfs_server aft-mtp-mount
- Connect your Android phone and put it in USB file transfer mode
- Mount the device: mount -t userlandfs -p 'aft-mtp-mount /boot/home/MyPhone -d -o use_ino' ~/MyPhone
- You can now access your phone data

Change-Id: Ic3efda7ffbc33737e6f4958428fb3ec9939ef105
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5198
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

userlandfs: add a pkg-config file for the fuse compatibility

This makes it a little simpler to build FUSE software with userlandfs.

Change-Id: I2a7e5494b7dc8db01e4c150de3fcdb99506ce119
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5197
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>