builtins to allow stdin to be closed

Prior to this fix, if stdin were explicitly closed, then builtins would
silently fail. For example:

    count <&-

would just fail with status 1. Remove this limitation and allow each
builtin to handle a closed stdin how it sees fit.

tests/checks/redirect.fish

@@ -50,7 +50,7 @@ test -f $tmpdir/file.txt && echo "File exists" || echo "File does not exist"
 
 function foo
     if set -q argv[1]
-        foo > $argv[1]
+        foo >$argv[1]
     end
     echo foo
 end
@@ -89,6 +89,19 @@ begin
 end 2>| cat >/dev/null
 #CHECK: is_stdout
 
+# Verify builtin behavior with closed stdin.
+# count silently ignores closed stdin, others may print an error.
+true <&-
+echo $status
+#CHECK: 0
+test -t 0 <&-
+echo $status
+#CHECK: 1
+read abc <&-
+#CHECKERR: read: stdin is closed
+
+
+
 # "Verify that pipes don't conflict with fd redirections"
 # This code is very similar to eval. We go over a bunch of fads
 # to make it likely that we will nominally conflict with a pipe