Update to reflect qthreads as the default tasking layer.
Merge branch 'master' of github.com:chapel-lang/chapel into doc-tasks-qthreads
More updates to reflect qthreads as default tasking.

README.chplenv:
- Put qthreads in front of fifo in list of tasking layers.
- Update description of default setting for CHPL_HWLOC.

README.executing:
- Update description of dataParTasksPerLocale default value.
- Update which tasking layers support -b and -t.

README.tasks:
- Swap order of qthreads and fifo tasking (qthreads first, as more
  important).
- Update stack overflow discussion to reflect setting execution-time
  default via compiler --[no-]stack-checks option, etc.  This leads to a
  bit of redundancy in the README, but too much information is better
  than too little, and we can improve this further when stack overflow
  checking is handled the same in all tasking layers.

README.cray:
- One adjustment to reflect default qthreads tasking.

README.localeModels:
- Do a little reformatting of too-long lines.
- Remove a now-superfluous mention of qthreads+hwloc being the default
  tasking layer with NUMA.
- Update Qthreads thread schedulers used for various CHPL_LOCALE_MODEL
  settings.
Merge pull request #500 from gbtitus/doc-tasks-qthreads

Update to reflect qthreads as the default tasking layer.

All of this is in one way or another related to the change to make qthreads our
default tasking implementation. The major changes are:
- Reorder to put qthreads descriptions before fifo ones, to minimize reading time
  for those using the expected/default configuration.
- Make adjustments for related configuration settings, such as the fact that with
  qthreads default we also now have hwloc enabled by default, and that we use
  different Qthreads thread schedulers with flat and NUMA locale models.
- Update some other things that were out of date, such as our description of the
  dataParTasksPerLocale default settings.
- Update the discussion of stack overflow detection, which wasn't as clear as it
  could be.

I also did a little bit of reformatting where some lines were too long.

Reviewed by Elliot.