Add initial support for fcntl and flock virtualization

[andriyolo]

Fcntl and flock are virtualized through VUFS using a supporting file for each lock request instead of the original one.

Note that this implementation needs testing:

• check that all cases are correctly handled
• check that the SC behave correctly in case of multi-threaded programs
• close the newly created files when the actual one is (it shouldn't be possible doing it before)