stdio: warn on suspicious uses

stdio: warn on suspicious uses

Using gets is almost ALWAYS wrong (it is extremely rare that you have
full control over stdin).  POSIX 2008 marked it as obsolete, even
though C89 requires it.  Attach a warning to remind developers.  Add
a comment to sprintf explaining why it does not get this treatment.

Improve the warnings for fseek/fseeko (and ftell/ftello), with
comments justifying our position.

Some of our unit tests never use large files, so rather than drag
in a dependency on fseeko, they should be the first compilation
units to use _GL_NO_LARGE_FILES.

* modules/stdio (Depends-on): Add warn-on-use.
(Makefile.am): Provide new substitutions.
* m4/stdio_h.m4 (gl_STDIO_H): Check for inline, ftello, and
fseeko.
* lib/stdio.in.h (gets): Always warn on use.
(fseek, ftell): Adjust when warnings are issued, and honor
_GL_NO_LARGE_FILES as a way to silence the warning.
* tests/test-fpurge.c [!GNULIB_FSEEK]: Use new means to squelch
any warning about large file offsets.
* tests/test-freadable.c [!GNULIB_FSEEK]: Likewise.
* tests/test-freading.c [!GNULIB_FSEEK]: Likewise.
* tests/test-fseeko.c [!GNULIB_FSEEK]: Likewise.
* tests/test-ftell.c [!GNULIB_FSEEK]: Likewise.
* tests/test-ftello.c [!GNULIB_FSEEK]: Likewise.
* tests/test-fwritable.c [!GNULIB_FSEEK]: Likewise.
* tests/test-fwriting.c [!GNULIB_FSEEK]: Likewise.
* tests/test-getopt.c [!GNULIB_FTELL]: Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>