If &debugmsg=1 is passed as part of the request parameters, display all

If &debugmsg=1 is passed as part of the request parameters, display all
the message keys instead of the actual messages.  I think this will be
very useful to sysops trying to find out what message in the MediaWiki
namespace to edit.  I hope this does not incur too much of a server
performance degradation, since this function is used pretty frequently.