Added support for using Squirrelmail without frames

Added support for using Squirrelmail without frames
(With many thanks to Jason Wickers (jwickers at scs.wsu.edu))
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There are still a few minor things to iron out, but overall
it works fairly well at this point.  Any plugin that calls
DisplayPageHeader() will now have to replace their </body></html>
with a simple call to noframes_bottom() to be compatible...
(Possibly rename that function to DisplayPageFooter in the future)

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@6225 7612ce4b-ef26-0410-bec9-ea0150e637f0