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This is actually several logical changes, but they are very difficult to
separate into individual commits because of how tangled the old macros
were.  Some of the changes are:

- general macro cleanup: do{}while(0) and whitespace issues
- add support for compiler-assisted thread-local storage (TLS)
- make MPID_Thread (e.g. pthread) TLS macros easier to understand and use
- drop non-portable parentheses from MPIU_ISTHREADED macro
- eliminate the MPID_CS_* family of macros which were simultaneously
  complementary to and in conflict with the newer MPIU_THREAD_CS_*
  macros

No reviewer.