libc_noux: style fix
test/libc_ffat: Operate on cwd instead of root dir

The test used to rely on a writable root directory. To reuse the test
for the new VFS, where '/' is never writable, I changed all absolute
paths to relative paths.
Move VFS from noux to 'os/include/vfs'

By separating the VFS from Noux, we become able to reuse the
VFS for libc-using programs. The most substantial change is the
removal of Noux::Sysio data structures from the VFS. Instead, the
Noux::Sysio refers to the VFS types now.

The new VFS library is located at 'os/include/vfs/'.

Furthermore, the patch removes various code duplications related
to pseudo file systems that provide a single file (block, zero,
random, terminal, null). The new 'Single_file_system' holds the
common boilerplate code for those.

Issue #999
vfs: LOG file system
libc: Add fd status and flags to File_descriptor

Those values need to be stored local to the FD. Including
them in the 'File_descriptor' eases the handling of libc
plugins.
libc: Make File_descriptor::fd_path char const *
Add libc_vfs plugin

Issue #999
Retire libc_log, use libc_vfs instead (WiP)