Converted Time counter to a uniform unsigned long it value.

Converted Time counter to a uniform unsigned long it value.
The initialized flag was redundant with the main Initialized flag,
and conversion of timeval to milliseconds in POSIX makes the code
cleaner. Timeval has a longer range, but the time value is already
limited by the GLUT API.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@706 7f0cb862-5218-0410-a997-914c9d46530a