move Portals SHMEM extensions into shmemx and add space for other extensions
s/ifdef USE_ORDERED_RMA/if ENABLE_RMA_ORDERING/g to utilize shmemconf.h
remove unnecessary flush ops
don't flush in fence/quiet only barrier if RMA ordered
s/SHEAP_BARRIERS/ENABLE_SHEAP_BARRIERS/g to match shmemconf.h
s/ENABLE_COMM_CACHING/(USE_)COMM_CACHING/g to match shmemconf.h
removed same_op_no_op stuff because MPI-3 is ambiguous (at best) about its usage
more skeletons for extensions
fix decl to match impl
s/USE_SMP_OPTIMIZATIONS/ENABLE_SMP_OPTIMIZATIONS/g