progs/cc-progress.c: Use `fstat' to discover the file size.

And `lseek' to discover the current offset.  Annoyingly, Android only
developed `ftello64' and `fseeko64' in API24, so we can't use these (and
it was a pretty grim circumlocution anyway).  On the other hand, Android
has had `lseek64' forever, and its `fstat' is natively 64-bit; and
there's no portability benefit to using the other functions because
Windows doesn't have them anyway.  (Indeed, `lseek' and `stat' are
ancient Unix, so probably more portable.)