Throw an error message if the user enters a bad username

For some reason kore isn't checking parameters anymore, so check
them lua-side.

src/lua/init.lua

@@ -513,6 +513,16 @@ function claim(req)
 		--Actually claim a name
 		http_request_populate_post(req)
 		local name = assert(http_argument_get_string(req,"user"))
+		--What in the world, Kore should be rejecting names that
+		--are not lower case & no symbols, but some still get through somehow.
+		if not name:match("^[a-z0-9]{1,30}$") then
+			print("Bad username:",name)
+			text = pages.claim{
+				err = "Usernames must match ^[a-z0-9]{1,30}$"
+			}
+			http_response(req,200,text)
+			return
+		end
 		local rngf = assert(io.open("/dev/urandom","rb"))
 		local passlength = string.byte(rngf:read(1)) + 64
 		local salt = rngf:read(64)