First approach to a helper setenv function for qthreads

Want a helper function for qthreads to setenvs because qthreads has two
varations of each env var QT_VAR and QTHREAD_VAR and it's easy to forget to
check both. This also adds warnings if we don't set because the qthreads env
vars were already set by a user.
Update helper routine and actually use it

This updates the chpl_qt_setenv function with some more debug info and sets
the QTHREAD_ version of the env var too (just incase the precedence of QT_
every changes in the qthreads source.)

Also adds a TODO for the CALL_STACK env var and changes our uses of setenv to
chpl_qt_setenv
Didn't quote setting "core"
Fix some typos and when we print override message

Now only prints override message we actually overrode, rather than just if we
were able to set something.
Make eitherSet a chpl_bool
Merge pull request #187 from ronawho/qthreads-setenv

Add a helper function to set a qthreads env var

[reviewed by @gbtitus]

This is meant to mirror setenv functionality, but qthreads has two environment
variables for every setting: a QT_ and a QTHREAD_ version. We often forget to
think about both so this wraps the overriding logic. In verbose mode it prints
out if we overrode values, or if we were prevented from setting values because
they existed (and override was 0.)

Note that unlike setenv it is a void function and doesn't check if the value
to set contains an "=" or if the value is NULL, however I can update it if that
seems useful. We previously just ignored any errors anyways and since this is
internal use only, it seems fine.

This also puts a note at STACK_SIZE so we don't forget that we want to update
it's behavior.

I have some upcoming changes that will change qthread env vars and this is
something I always forget about when setting a qthread env var and I think it
makes setting a qthread env var cleaner, even though the function is kind of
large for what it does.