Add fs module utility functions

[nbd168]

Implement flock() and ftruncate()

[jow-]

Hm, both new functions operate on file handles so they should become methods of fs.file. By current convention, all functions in the fs global namespace operate on paths, but not on open handles.

User who wish to use those functions with raw descriptor numbers might use:

let fd = 123;
let fp = fs.fdopen(fd);

fp.truncate();
fp.flock("x");

Or more concise:
fs.fdopen(123).truncate()

[nbd168]

Is it fine now?

[jow-]

Yes, LGTM!