Does the actual split.

• Is accessed with ?action=splittopics;sa=execute.
• Uses the main SplitTopics template.
• Supports three ways of splitting:
• (1) only one message is split off.
• (2) all messages after and including a given message are split off.
• (3) select topics to split (redirects to SplitSelectTopics()).
• Uses splitTopic function to do the actual splitting.
• Pretty much the same as SplitTopics2 in 1.0.x