Add ssl_set_arc4_support()

Rationale: if people want to disable RC4 but otherwise keep the default suite
list, it was cumbersome. Also, since it uses a global array,
ssl_list_ciphersuite() is not a convenient place. So the SSL modules look like
the best place, even if it means temporarily adding one SSL setting.
diff --git a/ChangeLog b/ChangeLog
index 255d359..a965f8d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -4,6 +4,10 @@
 Changes
    * Example programs for SSL client and server now disable SSLv3 by default.
 
+Features
+   * Add ssl_set_arc4_support() to make it easier to diable RC4 at runtime
+     while using the default ciphersuite list.
+
 = PolarSSL 1.3.9 released 2014-10-20
 Security
    * Lowest common hash was selected from signature_algorithms extension in