Provide separate calls for environmental and default call stack sizes.

Replace chpl_task_getMinCallStackSize() with separate calls to get the call
stack size from the environment (chpl_task_getEnvCallStackSize()) and the
default call stack size (chpl_task_getDefaultCallStackSize()).  This change
was mainly needed for qthreads, where we want a call stack size specified
by Chapel's CHPL_RT_CALL_STACK_SIZE environment variable to override any
specified by the Qthreads (QT|QTHREAD)_STACK_SIZE environment variable, but
we want the Qthreads environment variables to override the Chapel default.

I made a few other changes while I was here.  In chpl-tasks.c I did some
code cleanup.  In tasks-massivethreads.c, I clarified a comment and cleaned
up some white space to match their coding style.  And, I deleted these four
.execenv files, which all specified a stack size of 8000000 bytes, as far
as I could tell in order to deal with the tasking layer implementation-
dependent call stack sizes we used to have:
  test/distributions/robust/associative/basic/array_write.execenv
  test/distributions/robust/associative/basic/domain_write.execenv
  test/distributions/robust/associative/basic/whole_domain_assign.execenv
  test/parallel/cobegin/deitz/test_big_recursive_cobegin.execenv