libports: reuse libc_ffat as skeleton for fs tests

In addition, file-system tests will use the AHCI driver instead of the
ATAPI driver. This change side-steps certain issues with Qemu's IDE
emulation.

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.
libc: add Genode::Lock to File_descriptor class

This lock is needed to prevent multiple threads and/or multiple
libc-plugins from using the same file descriptor in p{read,write}.

Fixes #948.
libc: use File_descriptor::lock in pread/pwrite

Replace the static lock with the file descriptor lock because recursive
calls of p{read,write} would result in a deadlock when using multiple
libc-plugins at the same time.

Fixes #948.
scout: fix size calculation

Fixes #978.
base-nova: increase page fault handler stack size

When building Genode/NOVA with -O0, the page fault handler thread could
run out of stack.

Fixes #977.
os: initial preparation for 64Bit AHCI support

The AHCI driver seems to work on x86_64, though it should be treated
carefully because for now all adresses involved are <= 32Bit.