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/lib/libast/tm/tmxdate.c

@@ -836,7 +836,8 @@ tmxdate(register const char* s, char** e, Time_t now)
 			}
 			else
 			{
-				for (u = t; isspace(*u); u++);
+				for (u = t; isspace(*u); u++)
+					;
 				message((-1, "AHA#%d n=%d u=\"%s\"", __LINE__, n, u));
 				if ((j = tmlex(u, NiL, tm_info.format, TM_NFORM, tm_info.format + TM_SUFFIXES, TM_PARTS - TM_SUFFIXES)) >= 0 && tm_data.lex[j] == TM_PARTS)
 					s = u;
@@ -978,7 +979,8 @@ tmxdate(register const char* s, char** e, Time_t now)
 						}
 						continue;
 					}
-					for (s = t; skip[*s]; s++);
+					for (s = t; skip[*s]; s++)
+						;
 					message((-1, "AHA#%d s=\"%s\"", __LINE__, s));
 					if (*s == ':' || *s == '.' && ((set|state) & (YEAR|MONTH|DAY|HOUR)) == (YEAR|MONTH|DAY))
 					{