1) Set the stdio error indicator on __slbexpand() failure.
2) fgetwc(3) may succeed even when ferror(3) is already set
(for example, consider a program using SIG_IGN on SIGTTIN,
reading from the tty while in the background, getting EIO,
then coming to the foreground and reading again).
So do not force fgetwln(3) to fail in such a case either.
(Yes, the program should probably clearerr(3) before
reading again, but let's be nicer in case it forgets.)
getdelim works on strings up to SSIZE_MAX and returns -1 on EOF.
__fgetstr is now just a wrapper around __getdelim and ensures that
the buffer doesn't overflow the one provided by FILE.
__slbexpand is now static in fgetwln as it is the only consumer of that func.
