Additional cleanup around xterm-mouse

[Jared Finder]

Two more patches doing misc cleanup around xterm-mouse.

The first patch is very straightforward and should be trivial to review 
and merge.

I have a question about the right way to proceed with the second patch.  
Redirecting to read-key seems like the right solution here as it is 
"read-event but also take input-decode-map into account".  I needed to 
make a change to read-key's implementation to make it also return button 
down events and I saw two ways to do that.  I would like advice on which 
way is preferred.  In the patch you can see the two options commented in 
subr.el with Option 1 enabled by default.

Let me know what you think.

  -- MJF

0001-Migrate-usage-of-GPM_CLICK_EVENT-to-MOUSE_CLICK_EVEN.patch
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0002-Options-for-making-libraries-work-with-xt-mouse.patch
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