Symbolic CHPL_RT_NUM_THREADS_PER_LOCALE values: MAX_PHYSICAL and MAX_LOGICAL.

Add support for two symbolic values for CHPL_RT_NUM_THREADS_PER_LOCALE:
'MAX_PHYSICAL' and 'MAX_LOGICAL'.  These will be interpreted as the
number of physical and logical CPU instances available to the program,
respectively.  Also, add unit tests for these values and as part of
this, introduce a new section of tests/ to contain runtime-specific
tests.
Apply any comm layer maximum thread count.

If the comm layer imposes a maximum thread count, we should apply that
when we've got a symbolic CHPL_RT_NUM_THREADS_PER_LOCALE.
Merge pull request #429 from gbtitus/symbolic-numthreads-env-var

Support a couple of symbolic CHPL_RT_NUM_THREADS_PER_LOCALE values.

Add support for two symbolic values for CHPL_RT_NUM_THREADS_PER_LOCALE:
'MAX_PHYSICAL' and 'MAX_LOGICAL'. These will be interpreted as the
number of physical and logical CPU instances available to the program,
respectively. Also, add unit tests for these values and as part of
this, introduce a new section of tests/ to contain runtime-specific
tests.

Reviewed by Elliot.