iffe: fix broken shell detection; allow building on NetBSD 9.0

The shell detection test assumed that any shell that has $RANDOM
and isn't bash must be ksh and have the 'print' built-in command.
This broke iffe on NetBSD 9.0 sh, which has $RANDOM but not 'print'.

src/cmd/INIT/iffe.sh:
- Rewrite shell detection test to test for features actually used
  by iffe. "$shell" can now have the values 'bsh' for an ancient
  pre-POSIX Bourne shell, 'bash' for bash, 'ksh' for a shell with
  'let', 'typeset -u' and 'print' (ksh88, ksh93, mksh, pdksh, zsh),
  and 'posix' for another POSIX or POSIX-ish shell with modern
  command substitutions and parameter substitutions. (The 'posix'
  value currently is not actively checked for anywhere, but avoids
  unnecessary use of bsh fallbacks.)

src/cmd/INIT/iffe.sh

@@ -649,16 +649,19 @@ protoflags=
 puthdr=
 putlib=
 pragma=
-case $RANDOM in
-$RANDOM)shell=bsh
+case `eval 'v=NOposixNO; w=$(export "w=$v"; echo "${w%%NO}"); echo "${w##NO}"' 2>/dev/null` in
+posix)	shell=posix		# or at least POSIX-ish
+	case $BASH_VERSION in
+	[1-9]*)	shell=bash ;;
+	esac
+	case `eval 'PATH=/dev/null && let i=93-5 && typeset -u v=ksh$i && print -r - "$v"' 2>/dev/null` in
+	KSH88)	shell=ksh ;;	# also pdksh, mksh, zsh
+	esac
+	;;
+*)	shell=bsh		# ancient pre-POSIX Bourne shell
 	($executable .) 2>/dev/null || executable='test -r'
 	($exists .) 2>/dev/null || exists='test -r'
 	;;
-*)	case $BASH_VERSION in
-	?*)	shell=bash ;;
-	*)	shell=ksh ;;
-	esac
-	;;
 esac
 reallystatic=
 reallystatictest=