This function handles the interface for sending a new PM or replying to an existing one

• If $_REQUEST['pmsg'] is set, loads the information for that PM and inserts it in qoute tags in the text box
• Loads the InstantMessage template and language files
• Determines whether or not spell checking is available (based on the enableSpellChecking setting and the existence of pspell_new())
• Determines whether you're sending a new PM or quoting/replying to an existing one
• Loads up membernames for the ID(s) specified by $_REQUEST['u'], if it's set
• Sets up context variables and calls checkSubmitOnce to make sure you don't try to send the form more than once
• Accessed via index.php?action=pm;sa=send