portability: fix several issues with M4 subprocess.

M4's output pipe was not being drained upon fatal errors during
scan_skel.  As a result, broken-pipe messages from M4 were seen
on at least AIX, HP-UX, Solaris, and RHEL4, and this caused a
failure in the test suite.  The problem was that, on platforms
where the default disposition for SIGPIPE is ignore instead of
terminate, M4 sometimes saw fwrite fail with errno=EPIPE and
then reported it.  However, there's some sort of race condition,
because the new test group occasionally succeeded.
Reported by Albert Chin at
<http://lists.gnu.org/archive/html/bug-bison/2010-02/msg00004.html>.

There were also problems with the test suite livelocking on
Tru64 5.1b.  Reported by Didier Godefroy at
<http://lists.gnu.org/archive/html/bug-bison/2009-05/msg00005.html>.
Switching to create_pipe_bidi suggested by Akim Demaille.

To attempt to solve both of these problems, switch to gnulib's
create_pipe_bidi and register M4 process as a slave.  Along the
way, clean up file name conflict handling, which was affected by
the broken-pipe problem before the switch.
* NEWS (2.4.2): Document.
* THANKS (Didier Godefroy): Add.
* bootstrap.conf (gnulib_modules): Add pipe.
* gnulib: Update to latest to make sure we have all the latest
fixes.
* lib/local.mk (lib_libbison_a_SOURCES): Remove subpipe.h and
subpipe.c.
* po/POTFILES.in (lib/subpipe.c): Remove.
* src/files.c (compute_output_file_names): Update invocations
of output_file_name_check.
(output_file_name_check): In the case that the grammar file
would be overwritten, use complain instead of fatal, but replace
the output file name with /dev/null.  Use the /dev/null solution
for the case of two conflicting output files as well because it
seems safer in case Bison one day tries to open both files at
the same time.
* src/files.h (output_file_name_check): Update prototype.
* src/output.c (output_skeleton): Use create_pipe_bidi and
wait_subprocess.  Assert that scan_skel completely drains the
pipe.
* src/scan-skel.l (at_directive_perform): Update
output_file_name_check invocation.
* tests/output.at (AT_CHECK_CONFLICTING_OUTPUT): Check that the
grammar file actually isn't overwritten.
(Conflicting output files: -o foo.y): Update expected output.
* tests/skeletons.at (Fatal errors but M4 continues producing
output): New test group.
(cherry picked from commit 22cc8d813e)

Conflicts:

	NEWS
	bootstrap.conf
	lib/.cvsignore
	lib/.gitignore
	lib/Makefile.am
	m4/.cvsignore
	m4/.gitignore
	src/output.c

lib/.gitignore

@@ -11,6 +11,7 @@
 /asnprintf.c
 /basename-lgpl.c
 /basename.c
+/binary-io.h
 /bitrotate.h
 /c-ctype.c
 /c-ctype.h
@@ -18,6 +19,8 @@
 /c-strcasecmp.c
 /c-strncasecmp.c
 /charset.alias
+/cloexec.c
+/cloexec.h
 /config.charset
 /config.h
 /config.hin
@@ -25,6 +28,7 @@
 /dirname-lgpl.c
 /dirname.c
 /dirname.h
+/dup-safer-flag.c
 /dup-safer.c
 /dup2.c
 /errno.h
@@ -33,9 +37,12 @@
 /error.h
 /exitfail.c
 /exitfail.h
+/fatal-signal.c
+/fatal-signal.h
 /fcntl.c
 /fcntl.h
 /fcntl.in.h
+/fd-safer-flag.c
 /fd-safer.c
 /float+.h
 /float.h
@@ -48,6 +55,7 @@
 /frexpl.c
 /fseterr.c
 /fseterr.h
+/getdtablesize.c
 /getopt.c
 /getopt.h
 /getopt.in.h
@@ -83,7 +91,12 @@
 /memchr.valgrind
 /obstack.c
 /obstack.h
+/open.c
 /pipe-safer.c
+/pipe.c
+/pipe.h
+/pipe2-safer.c
+/pipe2.c
 /printf-args.c
 /printf-args.h
 /printf-frexp.c
@@ -97,18 +110,42 @@
 /quote.h
 /quotearg.c
 /quotearg.h
+/rawmemchr.c
+/rawmemchr.valgrind
 /realloc.c
 /ref-add.sed
 /ref-add.sin
 /ref-del.sed
 /ref-del.sin
+/sched.h
+/sched.in.h
+/sig-handler.h
+/sigaction.c
+/signal.h
+/signal.in.h
 /signbitd.c
 /signbitf.c
 /signbitl.c
+/sigprocmask.c
 /size_max.h
 /snprintf.c
+/spawn.h
+/spawn.in.h
+/spawn_faction_addclose.c
+/spawn_faction_adddup2.c
+/spawn_faction_addopen.c
+/spawn_faction_destroy.c
+/spawn_faction_init.c
+/spawn_int.h
+/spawnattr_destroy.c
+/spawnattr_init.c
+/spawnattr_setflags.c
+/spawnattr_setsigmask.c
+/spawni.c
+/spawnp.c
 /sprintf.c
 /stamp-h1
+/stat.c
 /stdbool.h
 /stdbool.in.h
 /stdbool_.h
@@ -127,6 +164,8 @@
 /stdlib.in.h
 /stdlib_.h
 /stpcpy.c
+/strchrnul.c
+/strchrnul.valgrind
 /streq.h
 /strerror.c
 /string.h
@@ -139,6 +178,13 @@
 /strtoul.c
 /strverscmp.c
 /strverscmp.h
+/sys
+/sys_stat.h
+/sys_stat.in.h
+/sys_wait.h
+/sys_wait.in.h
+/time.h
+/time.in.h
 /unistd--.h
 /unistd-safer.h
 /unistd.h
@@ -155,6 +201,9 @@
 /vfprintf.c
 /vsnprintf.c
 /vsprintf.c
+/w32spawn.h
+/wait-process.c
+/wait-process.h
 /wchar.h
 /wchar.in.h
 /wchar_.h