Another round of accumulated tweaks and cleanups

Notable changes:

src/cmd/ksh93/*.c:
- Get rid of all the dtuserdata(FOO,&sh,1) calls backported in
  cc492752. These set pointers to sh in Cdt objects. As of
  b590a9f1, the code does not use any pointers to sh, so these are
  superfluous.

src/cmd/ksh93/data/builtins.c,
src/cmd/ksh93/sh.1:
- As of ksh 93l 2001-06-01, the -h/trackall option has no effect at
  all, so trim its documentation.

src/lib/libast/man/stk.3,
src/lib/libast/man/stak.3:
- Correct the documentation on what the ST(A)K_SMALL option bit
  actually does based on a reading of the code.
- Document the STK_NULL option bit.

README.md,
src/cmd/ksh93/README:
- Add a note that -fdiagnostics-color=always will break the build.
  Ref.: https://github.com/ksh93/ksh/issues/379

src/lib/libast/Mamfile:
- Remove a 'rm -f astmath' command -- a file that is never created.
  But on Cygwin this removes astmath.exe, which *is* used. As a
  result, executing it failed on Cygwin, so the system incorrectly
  detected that Cygwin needs -lm for math functions.

src/cmd/ksh93/sh/main.c

@@ -174,7 +174,6 @@ int sh_main(int ac, char *av[], Shinit_f userinit)
 				/* preset aliases for interactive non-POSIX ksh */
 				dtclose(sh.alias_tree);
 				sh.alias_tree = sh_inittree(shtab_aliases);
-				dtuserdata(sh.alias_tree,&sh,1);
 			}
 		}
 #if SHOPT_REMOTE