Yet more accumulated tweaks and cleanups

Notable changes:

src/cmd/ksh93/bltins/trap.c: b_trap():
- Disable the unadvertised 'trap + SIG' feature in POSIX mode; it's
  not compatible as '+' is a legitimate command name.

src/cmd/ksh93/data/builtins.c:
- Give "pwd", "alarm" and "times" the BLT_ENV flag for better
  performance. There is no need for those to be stoppable.

src/cmd/ksh93/sh/xec.c:
- sh_eval(): Remove a "temporary tksh hack" and associated
  sh.fn_reset flag.
- sh_exec():
  - Remove now-used 'unpipe' flag and one remaining dead check for
    it (re: a2196f94, 4b22fd5d).
  - Fix unnecessary and confusing reuse of the 'type' variable.

src/lib/libast/comp/conf.sh:
- trap: Use 'rm -rf' instead of 'rm -f' to remove temp executables;
  on macOS, they may include *.dSYM directories.

src/cmd/ksh93/include/name.h

@@ -141,7 +141,7 @@ struct Ufunction
 #define BLT_ENV		(NV_RDONLY)		/* non-stoppable,
 						 * can modify environment */
 #define BLT_SPC		(NV_LJUST)		/* special built-ins */
-#define BLT_EXIT	(NV_RJUST)		/* exit value can be > 255 */
+#define BLT_EXIT	(NV_RJUST)		/* exit value can be > 255 or < 0 */
 #define BLT_DCL		(NV_TAGGED)		/* declaration command */
 #define BLT_NOSFIO	(NV_IMPORT)		/* doesn't use sfio */
 #define NV_OPTGET	(NV_BINARY)		/* function calls getopts */

src/cmd/ksh93/include/shell.h

@@ -337,7 +337,6 @@ struct Shell_s
 	struct argnod	*envlist;
 	struct dolnod	*arglist;
 	int		fn_depth;	/* scoped ksh-style function call depth */
-	int		fn_reset;
 	int		dot_depth;	/* dot-script and POSIX function call depth */
 	int		hist_depth;
 	int		xargmin;
@@ -381,14 +380,16 @@ struct Shell_s
 	Namfun_t	nvfun;
 	char		*mathnodes;
 	char		*bltin_dir;
-	struct Regress_s*regress;
 #if SHOPT_FILESCAN
 	char		*cur_line;
-#endif
+#endif /* SHOPT_FILESCAN */
 #if !SHOPT_DEVFD
 	char		*fifo;		/* FIFO name for current process substitution */
 	Dt_t		*fifo_tree;	/* for cleaning up process substitution FIFOs */
 #endif /* !SHOPT_DEVFD */
+#if SHOPT_REGRESS
+	struct Regress_s *regress;
+#endif /* SHOPT_REGRESS */
 };
 
 /* used for builtins */