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_test/ash-heredoc/heredoc4.tests

@@ -0,0 +1,3 @@
+cat <<EOF
+'$'
+EOF