Remove ability to turn off qthreads work stealing (for good this time)

The ability to turn off work stealing has been added and removed twice now.
Both times to try out the performance of the sherwood scheduler with work
stealing off. The first time it was turned off unconditionally and the second
only if an env var was set.

We are going to use the nemesis scheduler by default and only use sherwood for
numa now. I'm removing this not because we're using nemesis by default as it
might have been useful for numa but because it doesn't do what we expect it to.
It seems sherwood is highly intertwined with work stealing. I believe that when
you create tasks it places them all on a local shepherd and relies on work
stealing to distribute work (where as nemesis or something deals out work to
shepherds in a round robin fashion.) So turning off work stealing the way we
were didn't have the desired behavior.
Merge pull request #145 from ronawho/qthreads-remove-no-work-stealing

Remove ability to turn off qthreads work stealing (for good this time)

[trivial, not reviewed] 

The ability to turn off work stealing has been added and removed twice now.
Both times to try out the performance of the sherwood scheduler with work
stealing off. The first time it was turned off unconditionally and the second
only if an env var was set.

We are going to use the nemesis scheduler by default and only use sherwood for
numa now. I'm removing this not because we're using nemesis by default as it
might have been useful for numa but because it doesn't do what we expect it to.
It seems sherwood is highly intertwined with work stealing. I believe that when
you create tasks it places them all on a local shepherd and relies on work
stealing to distribute work (where as nemesis or something deals out work to
shepherds in a round robin fashion.) So turning off work stealing the way we
were didn't have the desired behavior.
Change the default value of fNoStackChecks

Stack checks are not really used currently, the variable and compiler flag were
added to support turning off qthread guard pages. While working on turning off
qthreads guard pages I noticed the default value was wrong. We want stack
checks on by default, and only want to turn them off if the user requested
no-stack-checks (or implicitly through --no-checks and --fast)
Disable qthread guard pages if user compiled with stack checks off

Turn off guard pages if the user turned off stack checks. That can happen
either explicitly through (--no-stack-checks) or implicitly through --no-checks
and thus --fast. Either way, CHPL_STACK_CHECKS (variable that gets generated to
chpl_compilation_config.h) gets set to 0. This adds an extern reference to that
variable and a check for it to determine if we should set qthread guard pages
off. Note that we do NOT override if the user explicitly set either of the
qthread guard pages env vars.
Disable qthread guard pages if user compiled with stack checks off

Turn off guard pages if the user turned off stack checks. That can happen
either explicitly through (--no-stack-checks) or implicitly through --no-checks
and thus --fast. Either way, CHPL_STACK_CHECKS (variable that gets generated to
chpl_compilation_config.h) gets set to 0. This adds an extern reference to that
variable and a check for it to determine if we should set qthread guard pages
off. Note that we do NOT override if the user explicitly set either of the
qthread guard pages env vars.