This change reworks ctest_start() so that simply calling
ctest_start(APPEND) will read all the information from the TAG file.
On top of that, it relaxes the argument parsing for ctest_start() to
allow greater flexibility in the argument ordering, and the documentation
for ctest_start() has been cleaned up.
In the ctest(1) manual and ctest_start() command documentation, convert
mentions of CTest Script variables to cross-references so they will be
rendered as links.
