libports: reuse libc_ffat as skeleton for fs tests

Fixes #942.
libports: add FUSE implementation

This FUSE implementation consists of libfuse, which provides a
subset of the FUSE 2.6 API and libc_fuse, which provides support
for accessing FUSE based file system via the libc.

Fixes #942.
libports: add fuse-exfat

Fixes #942.
libports: add fuse-ext2

Fixes #942.