Break up TRIBITS_GET_ENABLED_LIST() into for specific functions (TriBITS #38)

The problem with the old TRIBITS_GET_ENABLED_LIST() function is that it only
worked for explicit "ON" and "OFF".  That caused a problem when people used
TRUE and FALSE (see TriBITS #38).

Using these new functions is clearer and should also be faster because the if
statements in the inner loops are much simpler.

I have not added tests to prove that TRUE and FALSE work but all of the
existing tests all pass.
Removing explicit if logic for OFF or ON for enable vars (TriBITS #38)

All of the if statements that used to check STREQUAL ON or OFF should now all
correctly work for other valid values for true and false in CMake.

Now to add a test that proves this works for TRUE and FALSE.
Adding test for usage of TRUE/FASLE for enable vars (TriBITS #38)

This test shows that TriBITS now accepts TRUE and FALSE for the enable vars
and not just ON and OFF.

Build/Test Cases Summary
Enabled Packages: TriBITS
Enabled all Forward Packages
0) MPI_DEBUG => passed: passed=156,notpassed=0 (2.28 min)
1) SERIAL_RELEASE => passed: passed=156,notpassed=0 (2.10 min)
Other local commits for this build/test group: 990afb3, 6ef600f, a5cada6, 4feb929, 09ed04c