lseek was triggering a number overflow by converting long to integer on
its return value

Added specific lseek test

 - improved statfs system call
 - intercepted statvfs library call

Both of the above function now report the aggregated real size
of all the data-nodes.

You can test this with `df "/tmp/gekkofs_mountpoint"` command

 - Added adafs_pwrite and adafs_pread functions:
        they accept an OpenFile pointer instead of a fd so that can be
        called by function that already accessed the OpenFileMap.

 - Moved the writev function into internal adafs functions.

 - Added internal adafs_write function:
        instead messing up with the file position at the interception
        layer, we can use this internal function that hides the update of the
        file position two.

Can be used to retrieve the Metadata object for a named path

In several places the adafs_stat function have been replaced with this
new adafs_metadata. This allowed to avoid the construnction of the
additional `struct stat`.

Instead of using the more complex adafs_pread function now is possible
to use adafs_read when it is necessary to read starting from the current
file position

Implemented all the logic to handle truncate operation.

Test: added truncate test

Support for the following calls has been added:
 - opendir
 - readdir
 - closedir

The readdir call use the new RPC get_dirents that ask to all the
nodes (broadcast) about all the existing first-level entries
of a specific directory.

ADA-FS's statfs will fill the struct mostly with rough estimations.
It is not supposed to return accurate data but "some" data for applications to work

Added lseek64 syscall and used file flags properly

- Add: adafs_access, open with check if object exists
- Add: root metadentry to database of each node
- Modified: Open, unlink, mkdir, rmdir and combined their functionality to "create node" and "remove node"