win32: updating keyboard down/up handling. Split off to own function that deals with...

win32: updating keyboard down/up handling. Split off to own function that deals with both down and up to reduce code duplication. Also only get async keystate to see if left or right alt, control, shift is pressed/released when a press on one of these is detected in the first place, not with every message that comes in.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1505 7f0cb862-5218-0410-a997-914c9d46530a