Fix spurious export attribute when printing compound variables

Reproducer script:

    typeset -Ttyp1 typ1=(
            function get {
                    .sh.value="'Sample'";
            }
    )
    typ1 var11
    typeset -p .sh.type
    typeset -p .sh.type

Buggy output:

    namespace sh.type
    {
            typeset -r typ1='Sample'
    }
    namespace sh.type
    {
            typeset -x -r typ1='Sample'
    }

An -x (export) attribute is magically pulled out of a hat.

Analysis: The walk_tree() function in nvdisc.c repurposes (!) the
NV_EXPORT attribute as an instruction to turn *off* indenting when
pretty-printing the values of compound variables. The
print_namval() function in typeset.c, implementing 'typeset -p',
turns on NV_EXPORT for compound variables to inhibit indentation.
But it then does not bother to turn it off, which causes this bug.

src/cmd/ksh93/bltins/typeset.c: print_namval():
- When printing compound variables, only turn on NV_EXPORT
  temporarily.

Resolves: https://github.com/ksh93/ksh/issues/456

src/cmd/ksh93/tests/types.sh

@@ -734,5 +734,42 @@ exp=8
 [[ $got == $exp ]] || err_exit "short integer arrays in types isn't working correctly" \
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 
+# ======
+# Printing .sh.type should not crash or give variables a spurious -x attribute.
+# https://github.com/ksh93/ksh/issues/456
+
+got=$("$SHELL" -c 'typeset -p .sh.type
+typeset -Ttyp1 typ1=(
+	function get {
+	        .sh.value="'\''Sample'\''";
+	}
+)
+typeset -p .sh.type
+typ1
+command typ1 var11
+typ1
+print $var11
+print -v var11
+typeset -p .sh.type' 2>&1)
+
+exp='namespace sh.type
+{
+	:
+}
+namespace sh.type
+{
+	typeset -r typ1='\''Sample'\''
+}
+typ1 var11='\''Sample'\''
+'\''Sample'\''
+'\''Sample'\''
+namespace sh.type
+{
+	typeset -r typ1='\''Sample'\''
+}'
+
+[[ $got == "$exp" ]] || err_exit "'tyeset -p .sh.type' failed" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))