Even better emulate POSIX file functions

Added small fixes and workarounds in VFS, a number of fixes in emulated functions.
NOTE: emulated functions require a huge refactoring/update/rewrite, this is the minimal set of fixes. Last commit can be discarded, but I don't think that it can break anything in broken emulation (support for any VFS without Seek() was fully broken).
They are logically grouped.
I don't think that individual comments are required. Can add comment: `check the errno is correct according to POSIX specifications`