engine: resources: Add an http:flag resource

This adds a new http:flag resource which can autogroup into an
http:server resource to receive actions from client HTTP requests, and
forward these values on to other resources.

examples/lang/http-flag0.mcl

@@ -0,0 +1,37 @@
+import "value"
+
+http:server ":8080" {	# by default http uses :80 but using :8080 avoids needing root!
+	#address => ":8080",	# you can override the name like this
+	#timeout => 60,		# add a timeout (seconds)
+}
+
+# you can add a raw file like this...
+http:file "/file1" {
+	data => "hello, world, i'm file1 and i don't exist on disk!\n",
+}
+
+# wget --post-data 'key=hello&whatever=bye' -O - http://127.0.0.1:8080/flag1
+http:flag "/flag1" {
+	#server => ":8080",
+	key => "key",
+}
+value "value1" {
+	any => "tmp",	# can be any type
+}
+print "print1" {
+	Meta:autogroup => false,
+}
+
+Http:Flag["/flag1"].value -> Print["print1"].msg
+Http:Flag["/flag1"].value -> Value["value1"].any
+
+$ret = value.get_str("value1")	# name of value resource
+$val = $ret->value
+test "get" {
+	anotherstr => $val,
+	onlyshow => ["AnotherStr",], # displays nicer
+}
+file "/tmp/file" {
+	state => $const.res.file.state.exists,
+	content => "value: ${val}\n",
+}