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/.cvsignore

@@ -8,6 +8,7 @@ argmatch.h
 asnprintf.c
 basename-lgpl.c
 basename.c
+binary-io.h
 bitrotate.h
 c-ctype.c
 c-ctype.h
@@ -15,6 +16,8 @@ c-strcase.h
 c-strcasecmp.c
 c-strncasecmp.c
 charset.alias
+cloexec.c
+cloexec.h
 config.charset
 config.h
 config.hin
@@ -22,6 +25,7 @@ configmake.h
 dirname-lgpl.c
 dirname.c
 dirname.h
+dup-safer-flag.c
 dup-safer.c
 dup2.c
 errno.h
@@ -30,9 +34,12 @@ error.c
 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
@@ -45,6 +52,7 @@ frexp.c
 frexpl.c
 fseterr.c
 fseterr.h
+getdtablesize.c
 getopt.c
 getopt.h
 getopt.in.h
@@ -80,7 +88,12 @@ memchr.c
 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
@@ -94,18 +107,42 @@ quote.c
 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
@@ -124,6 +161,8 @@ stdlib.h
 stdlib.in.h
 stdlib_.h
 stpcpy.c
+strchrnul.c
+strchrnul.valgrind
 streq.h
 strerror.c
 string.h
@@ -136,6 +175,13 @@ strtol.c
 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
@@ -152,6 +198,9 @@ verify.h
 vfprintf.c
 vsnprintf.c
 vsprintf.c
+w32spawn.h
+wait-process.c
+wait-process.h
 wchar.h
 wchar.in.h
 wchar_.h