[BZ 1190] Make EOF sticky in stdio.

[BZ 1190] Make EOF sticky in stdio.

C99 specifies that the EOF condition on a file is "sticky": once EOF
has been encountered, all subsequent reads should continue to return
EOF until the file is closed or something clears the "end-of-file
indicator" (e.g. fseek, clearerr).  This is arguably a change from
C89, where the wording was ambiguous; the BSDs always had sticky EOF,
but the System V lineage would attempt to read from the underlying fd
again.  GNU libc has followed System V for as long as we've been
using libio, but nowadays C99 conformance and BSD compatibility are
more important than System V compatibility.

You might wonder if changing the _underflow impls is sufficient to
apply the C99 semantics to all of the many stdio functions that
perform input.  It should be enough to cover all paths to _IO_SYSREAD,
and the only other functions that call _IO_SYSREAD are the _seekoff
impls, which is OK because seeking clears EOF, and the _xsgetn impls,
which, as far as I can tell, are unused within glibc.

The test programs in this patch use a pseudoterminal to set up the
necessary conditions.  To facilitate this I added a new test-support
function that sets up a pair of pty file descriptors for you; it's
almost the same as BSD openpty, the only differences are that it
allocates the optionally-returned tty pathname with malloc, and that
it crashes if anything goes wrong.

	[BZ #1190]
        [BZ #19476]
	* libio/fileops.c (_IO_new_file_underflow): Return EOF immediately
	if the _IO_EOF_SEEN bit is already set; update commentary.
	* libio/oldfileops.c (_IO_old_file_underflow): Likewise.
	* libio/wfileops.c (_IO_wfile_underflow): Likewise.

	* support/support_openpty.c, support/tty.h: New files.
	* support/Makefile (libsupport-routines): Add support_openpty.

	* libio/tst-fgetc-after-eof.c, wcsmbs/test-fgetwc-after-eof.c:
	New test cases.
	* libio/Makefile (tests): Add tst-fgetc-after-eof.
	* wcsmbs/Makefile (tests): Add tst-fgetwc-after-eof.