Update command-line options, script variables, and documentation to use
the term "group" instead of "track". The old terms are still available
for now, but they are now undocumented.
This makes our terminology more consistent with CDash. The goal of this
change is to make it more clear to our users how CTest and CDash interact
with each other.
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.
