[FIXED JENKINS-9915] Make captcha support optional; remove LGPL jcaptcha

This change is largely based on changes made by Winston Prakash for Hudson.
The main details about his changes can be found here:

http://issues.hudson-ci.org/browse/HUDSON-8844

I deviated a bit from his changes where it was appropriate (e.g. the
description for allow-signup.html was just plain wrong). Also, there
was a lot of collateral damage from formatting in his changeset, which
I have factored out.

I tested the basic functionality of this change and it seems to be OK.

core/src/main/java/hudson/Functions.java

@@ -34,6 +34,7 @@ import hudson.security.AccessControlled;
 import hudson.security.AuthorizationStrategy;
 import hudson.security.Permission;
 import hudson.security.SecurityRealm;
+import hudson.security.captcha.CaptchaSupport;
 import hudson.security.csrf.CrumbIssuer;
 import hudson.slaves.Cloud;
 import hudson.slaves.ComputerLauncher;
@@ -687,6 +688,10 @@ public class Functions {
         return ParameterDefinition.all();
     }
 
+    public static List<Descriptor<CaptchaSupport>> getCaptchaSupportDescriptors() {
+        return CaptchaSupport.all();
+    }
+
     public static List<Descriptor<ViewsTabBar>> getViewsTabBarDescriptors() {
         return ViewsTabBar.all();
     }