Make 'set -ql' search up to function scope

Previously 'set -ql' would only look for variables in the
immediate local scope. This was not very useful. It's also
arguably surprising, since a 'set -l' in a function, followed
by a 'set -ql' in a child block, would fail. There was also no
way to check for a function-scoped variable, since omitting the
scope would also pull in global variables.

We could revisit this and introduce an explicit function scope.

Fixes #2502

tests/test4.in

@@ -177,3 +177,14 @@ true ; set -NOT_AN_OPTION 2> /dev/null ; echo 9 $status # no passthrough
 false ; set foo (echo A; true) ; echo 10 $status $foo
 true ; set foo (echo B; false) ; echo 11 $status $foo
 true
+
+echo "Verify set -ql behavior" # see 2502
+function setql_check
+  set -l setql_foo val
+  if set -ql setql_foo
+    echo Pass
+  else
+    echo Fail
+  end
+end
+setql_check

tests/test4.out

@@ -34,3 +34,5 @@ Verify that set passes through exit status, except when passed -n or -q or -e
 9 1
 10 0 A
 11 1 B
+Verify set -ql behavior
+Pass