Fix lseek on mingw.

Fix lseek on mingw.
* modules/lseek: New module.
* m4/lseek.m4: New file.
* lib/lseek.c: New file.
* modules/lseek-tests: New file.
* tests/test-lseek.c: New file.
* tests/test-lseek.sh: New file.
* MODULES.html.sh: Document lseek module.
* modules/fflush (Depends-on): Add lseek, fseeko.
* modules/fseeko (Depends-on): Likewise.
* modules/ftello (Depends-on): Likewise.
* m4/fseeko.m4 (gl_FUNC_FSEEKO): Replace fseek[o] if lseek is
broken.
* m4/ftello.m4 (gl_FUNC_FTELLO): Replace ftell[o] if lseek is
broken.
* m4/fflush.m4 (gl_REPLACE_FFLUSH): Trigger fseeko module.
* lib/fseeko.c (rpl_fseeko): Quit early on non-seekable files.
* lib/ftello.c (rpl_ftello): Likewise.
* tests/test-fseeko.c (main): Test this.
* tests/test-fseeko.sh: Likewise.
* tests/test-ftello.c (main): Likewise.
* tests/test-ftello.sh: Likewise.
* lib/stdio_.h (fseek, ftell): Simplify, since missing fseeko now
implies replacing fseek.
* modules/stdio (Makefile.am): No longer need HAVE_FSEEKO,
HAVE_FTELLO.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add lseek info.
* modules/unistd (Makefile.am): Likewise.
* lib/unistd_.h (lseek): Declare a replacement.
* doc/functions/lseek.texi (lseek): Document this fix.
* doc/functions/fseek.texi (fseek): Likewise.
* doc/functions/ftell.texi (ftell): Likewise.