Its API in inherently non-threadsafe (and it depends on certain properties
of the underlying stdio implementation), so it shouldn't be a first-class
function for general use, polluting the application namespace.
Also remove the FLOCKFILE() code from it - this is under control of the
calling function now.
(XXX I'm not sure whether the FLOCKFILE() should be kept in fgetln(),
this function cannot be used in multiple threads anyway. It doesn't
hurt much, and it might prevent corruption of internal FILE structures.)
