Fix many compiler warnings and remove unused variables (#191)

Most of these changes remove unused variables, functions and labels
to fix -Wunused compiler warnings. Somewhat notable changes:

src/cmd/ksh93/bltins/print.c:
- Removed the unused 'neg' variable.
  Patch from ksh2020: https://github.com/att/ast/pull/725

src/cmd/ksh93/bltins/sleep.c:
- Initialized ns to fix three -Wsometimes-uninitialized warnings.

src/cmd/ksh93/edit/{emacs,vi}.c:
- Adjust strncpy size to fix two -Wstringop-truncation warnings.

src/cmd/ksh93/include/shell.h:
- The NOT_USED macro caused many -Wunused-value warnings,
  so it has been replaced with ksh2020's macro:
  19d0620a

src/cmd/ksh93/sh/expand.c:
- Removed an unnecessary 'ap = ' since 'ap' is never read
  between stakseek and stakfreeze.

src/cmd/ksh93/edit/vi.c: refresh():
- Undef this function's 'w' macro at the end of it to stop it
  potentially interfering with future code changes.

src/cmd/ksh93/sh/nvdisc.c,
src/lib/libast/misc/magic.c,
src/lib/libast/regex/regsubexec.c,
src/lib/libast/sfio/sfpool.c,
src/lib/libast/vmalloc/vmbest.c:
- Fixed some indentation to silence -Wmisleading-indentation
  warnings.

src/lib/libast/include/ast.h:
- For clang, now only suppress hundreds of -Wparentheses warnings
  as well as a few -Wstring-plus-int warnings.
  Clang's -Wparentheses warns about things like
  	if(foo = bar())
  which assigns to foo and checks the assigned value.
  Clang wants us to change this into
  	if((foo = bar()))
  Clang's -Wstring-plus-int warns about things like
  	"string"+x
  where x is an integer, e.g. "string"+3 represents the string
  "ing". Clang wants us to change that to
  	"string"[3]
  The original versions represent a perfectly valid coding style
  that was common in the 1980s and 1990s and is not going to change
  in this historic code base. (gcc does not complain about these.)

Co-authored-by: Martijn Dekker <martijn@inlv.org>

src/cmd/ksh93/include/shell.h

@@ -37,8 +37,17 @@
 #   include	<nval.h>
 #endif /* _SH_PRIVATE */
 
+/*
+ * This is a macro that can be used to silence "unused parameter" warnings from the compiler for
+ * functions which need to accept parameters they do not use because they need to be compatible
+ * with an interface.
+ */
 #undef NOT_USED
-#define NOT_USED(x)	(&x,1)
+#define NOT_USED(expr)		\
+	do {			\
+		(void)(expr);	\
+	} while (0)
+
 
 /* options */
 typedef struct