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/Makefile.am (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
	m4/.cvsignore
	m4/.gitignore
	src/output.c

m4/.gitignore

@@ -3,6 +3,7 @@
 /alloca.m4
 /argmatch.m4
 /assert.m4
+/cloexec.m4
 /config-h.m4
 /dirname.m4
 /dos.m4
@@ -16,6 +17,7 @@
 /exponentf.m4
 /exponentl.m4
 /extensions.m4
+/fatal-signal.m4
 /fcntl-o.m4
 /fcntl.m4
 /fcntl_h.m4
@@ -25,6 +27,7 @@
 /fprintf-posix.m4
 /frexp.m4
 /frexpl.m4
+/getdtablesize.m4
 /getopt.m4
 /getpagesize.m4
 /gettext.m4
@@ -63,10 +66,15 @@
 /mbswidth.m4
 /memchr.m4
 /mmap-anon.m4
+/mode_t.m4
 /multiarch.m4
 /nls.m4
 /nocrash.m4
+/open.m4
+/pipe.m4
+/pipe2.m4
 /po.m4
+/posix_spawn.m4
 /printf-frexp.m4
 /printf-frexpl.m4
 /printf-posix-rpl.m4
@@ -75,12 +83,20 @@
 /progtest.m4
 /quote.m4
 /quotearg.m4
+/rawmemchr.m4
 /realloc.m4
+/sched_h.m4
 /setenv.m4
+/sig_atomic_t.m4
+/sigaction.m4
+/signal_h.m4
+/signalblocking.m4
 /signbit.m4
 /snprintf-posix.m4
 /snprintf.m4
+/spawn_h.m4
 /sprintf-posix.m4
+/stat.m4
 /stdbool.m4
 /stddef_h.m4
 /stdint.m4
@@ -89,6 +105,7 @@
 /stdio_h.m4
 /stdlib_h.m4
 /stpcpy.m4
+/strchrnul.m4
 /strerror.m4
 /string_h.m4
 /strndup.m4
@@ -96,7 +113,10 @@
 /strtol.m4
 /strtoul.m4
 /strverscmp.m4
+/sys_stat_h.m4
+/sys_wait_h.m4
 /threadlib.m4
+/time_h.m4
 /unistd-safer.m4
 /unistd_h.m4
 /unlocked-io.m4
@@ -105,6 +125,7 @@
 /vsnprintf-posix.m4
 /vsnprintf.m4
 /vsprintf-posix.m4
+/wait-process.m4
 /warn-on-use.m4
 /warning.m4
 /warnings.m4