ash: bash-compatible $'...' shouldn't expand in double quotes

Bash doesn't expand its $'...' construct in double quotes:

   $ echo "$'a\tb'"
   $'a\tb'

Change BusyBox ash to do the same.  This also fixes a problem with
here documents where BusyBox ash gave an incorrect result for:

   $ cat <<EOF
   > '$'
   > EOF
   '$'

Reported-by: Timo Teras <timo.teras@iki.fi>
Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash.c

@@ -11500,7 +11500,7 @@ parsesub: {
 	 || (c != '(' && c != '{' && !is_name(c) && !is_special(c))
 	) {
 #if ENABLE_ASH_BASH_COMPAT
-		if (c == '\'')
+		if (syntax != DQSYNTAX && c == '\'')
 			bash_dollar_squote = 1;
 		else
 #endif