stdbuf: A new program to run a command with modified stdio buffering

stdbuf: A new program to run a command with modified stdio buffering

* AUTHORS: Register as the author.
* NEWS: Mention this change.
* README: Add stdbuf command to list.
* configure.ac: Only enable on ELF systems with GCC.
* cfg.mk (sc_system_h_headers): Use VC_LIST_EXCEPT rather than
VC_LIST, so we can add an exception, if needed.
* .x-sc_system_h_headers: New file.  Exempt libstdbuf.c.
* Makefile.am (syntax_check_exceptions): Add .x-sc_system_h_headers.
* doc/coreutils.texi (stdbuf invocation): Add stdbuf info.
* man/.gitignore: Ignore generated manpage.
* src/.gitignore: Ignore stdbuf and libstdbuf.so binaries.
* man/Makefile.am (stdbuf.1): Add dependency.
* man/stdbuf.x: New file with example usage.
* po/POTFILES.in: Reference new command and shared library sources.
* src/Makefile.am (build_if_possible__progs): Add stdbuf and libstdbuf,
(pkglib_PROGRAMS): Reference optional shared lib,
(libstdbuf_so_LDADD): Ensure we don't link with non PIC libcoreutils.a.
(libstdbuf_so_LDFLAGS): Add -shared GCC option,
(libstdbuf_so_CFLAGS): Add -fPIC GCC option.
(check-README): Exclude libstbuf.
(check-AUTHORS): ditto.
(sc_tight_scope): Exclude functions starting with __.
* src/libstdbuf.c: The LD_PRELOAD shared library to control buffering.
* src/stdbuf.c: New file to setup env variables before execing command.
* tests/Makefile.am: Reference new test file.
* tests/misc/help-version: Set expected exit codes.
* tests/misc/invalid-opt: ditto.
* tests/misc/stdbuf: Add 9 tests.