- Because my_seek actually is capable of returning an error code we should
  exploit that in the best possible way.
- There might be kernel errors or other errors we can't predict and capturing
  the return value of all system calls gives us better understanding of
  possible errors.


mysys/mf_iocache.c:
  - Added check on return value for my_seek
  - Added comments
mysys/my_chsize.c:
  - Added check on return value for my_seek
  - Added comments
mysys/my_lock.c:
  - Added check on return value for my_seek
  - Added comments
mysys/my_seek.c:
  - Added comments