WIP: Use sigaction to fix multiple test failures under ASan

This commit fixes multiple test failures when the coprocess, io and pty
tests are run under ASan. Unlike the ksh93v- approach of implementing
sigaction via a sh_signal function, this commit repurposes the
(otherwise unused) signal wrapper function in libast, which already uses
sigaction for signal handling.

src/lib/libast/features/map.c:
- Use the libast signal function if sigaction is present.
- Add necessary sig.h include (the Mamfile was also updated to account
  for this).

src/lib/libast/Mamfile

@@ -114,6 +114,13 @@ make install
 											done FEATURE/options generated
 											prev FEATURE/mmap
 											prev FEATURE/lib
+											make sig.h implicit
+												make FEATURE/sig
+													prev features/sig.sh
+													exec - iffe ${IFFEFLAGS} -v -X ast -X std -c "${CC} ${mam_cc_FLAGS} ${CCFLAGS} ${LDFLAGS}" run features/sig.sh
+												done FEATURE/sig generated
+												exec - ${STDCP} -f FEATURE/sig sig.h
+											done sig.h generated
 										done features/map.c
 										exec - iffe ${IFFEFLAGS} -v -X ast -X std -c "${CC} ${mam_cc_FLAGS} ${CCFLAGS} -I.  ${LDFLAGS}" run features/map.c
 									done FEATURE/map generated
@@ -1109,13 +1116,7 @@ make install
 			make signal.o
 				make misc/signal.c
 					prev ast_map.h
-					make sig.h implicit
-						make FEATURE/sig
-							prev features/sig.sh
-							exec - iffe ${IFFEFLAGS} -v -X ast -X std -c "${CC} ${mam_cc_FLAGS} ${CCFLAGS} ${LDFLAGS}" run features/sig.sh
-						done FEATURE/sig generated
-						exec - ${STDCP} -f FEATURE/sig sig.h
-					done sig.h generated
+					prev sig.h
 					prev include/ast.h
 				done misc/signal.c
 				exec - ${CC} ${mam_cc_FLAGS} ${CCFLAGS} -I. -Icomp -Iinclude -Istd -c misc/signal.c

src/lib/libast/features/map.c

@@ -2,7 +2,7 @@
 *                                                                      *
 *               This software is part of the ast package               *
 *          Copyright (c) 1985-2011 AT&T Intellectual Property          *
-*          Copyright (c) 2020-2023 Contributors to ksh 93u+m           *
+*          Copyright (c) 2020-2024 Contributors to ksh 93u+m           *
 *                      and is licensed under the                       *
 *                 Eclipse Public License, Version 2.0                  *
 *                                                                      *
@@ -31,6 +31,7 @@
 #include "FEATURE/vmalloc"
 #include "FEATURE/eaccess"
 #include "FEATURE/api"
+#include <sig.h>
 
 #if _opt_map_libc && !defined(_map_libc)
 #define _map_libc	1
@@ -134,6 +135,11 @@ main(void)
 	printf("#define glob		_ast_glob\n");
 	printf("#undef	globfree\n");
 	printf("#define globfree	_ast_globfree\n");
+	/* use the libast signal() function (aka sigaction) when applicable */
+#if _map_libc || (!_std_signal && _lib_sigaction && defined(SA_NOCLDSTOP))
+	printf("#undef	signal\n");
+	printf("#define signal      	_ast_signal\n");
+#endif
 #if _map_libc
 	printf("#undef	memdup\n");
 	printf("#define memdup		_ast_memdup\n");
@@ -293,8 +299,6 @@ main(void)
 	printf("#define setenviron      _ast_setenviron\n");
 	printf("#undef	sigcritical\n");
 	printf("#define sigcritical      _ast_sigcritical\n");
-	printf("#undef	signal\n");
-	printf("#define signal      	_ast_signal\n");
 	printf("#undef	sigunblock\n");
 	printf("#define sigunblock      _ast_sigunblock\n");
 	printf("#undef	stracmp\n");