Consistently represent null pointers with NULL

[JohnoKing]

This relatively unimportant pull request implements the following changes:

• Fixed 3,427+ -Wzero-as-null-pointer-constant warnings when compiling with Clang 22.1.8. This is primarily for code style consistency (previous usage of 0 and NULL was seemingly random).
  • Switched some struct initialization to C99 style incidentally to also fix some -Wzero-as-null-pointer-constant warnings. Progresses Migrate code to C99 #777
• Fixed a few variable typing nits in the conf.tab tests; also removed some fallback test code for IRIX.

[McDutchie]

Nice; thanks. At long last we're rid of the 0/NULL annoyance.

I'll need to verify that this builds on Solaris 11.4 with Solaris Studio cc; I seem to recall it doesn't like the use NULL (defined as ((void*)0)) for function pointers.

[McDutchie]

Ah ha, I forgot I already took care of that three years ago (commit 0309d74) in features/standards, the results of which get included everywhere.

ksh/src/lib/libast/features/standards

Lines 143 to 151 in efc39a2

	/*
	* Though POSIX says it must be allowed, Solaris Studio cc dislikes NULL, a.k.a.
	* (void*)0, being used for function pointers. It warns, or in some cases it even
	* throws an error. Just use 0 for NULL, as that is always acceptable in C.
	*/
	#if __SUNPRO_C
	#undef NULL
	#define NULL 0
	#endif /* __SUNPRO_C */

[McDutchie]

I'm getting a build error with this PR:

# src/cmd/ksh93/Mamfile: 1027-1030: make init.o
+ cc -D_BLD_ksh -O0 -g -funsigned-char -Wall -Wsign-compare -Wshorten-64-to-32 -Wsign-conversion -Wimplicit-int-conversion -fno-strict-aliasing '-DSH_DICT="libshell"' -I. -I/usr/local/src/ksh93/ksh/src/cmd/ksh93 -Iinclude -I/usr/local/src/ksh93/ksh/src/cmd/ksh93/include -I/usr/local/src/ksh93/ksh/arch/darwin.arm64-64,d/include/ast '-D_API_ast=20100309' -c /usr/local/src/ksh93/ksh/src/cmd/ksh93/sh/init.c
/usr/local/src/ksh93/ksh/src/cmd/ksh93/sh/init.c:2036:24: warning: pointer/integer type mismatch in conditional expression ('wctrans_t' (aka 'int') and 'void *') [-Wconditional-type-mismatch]
 2036 |         wctrans_t       trans = name?wctrans(name):NULL;
      |                                     ^~~~~~~~~~~~~~ ~~~~
/usr/local/src/ksh93/ksh/src/cmd/ksh93/sh/init.c:2036:12: error: incompatible pointer to integer conversion initializing 'wctrans_t' (aka 'int') with an expression of type 'void *' [-Wint-conversion]
 2036 |         wctrans_t       trans = name?wctrans(name):NULL;
      |                         ^       ~~~~~~~~~~~~~~~~~~~~~~~
1 warning and 1 error generated.