support new HG_Init_opt2() function in Mercury

[carns]

See mercury-hpc/mercury#677. There is a new function in Mercury to support updated init info structs without breaking binary compatibility in the previous HG_Init_opt() function.

Margo will need a configure test to check for the presence of this function and use it if available.

The original function still works, but will not support new fields in the init info function.

@soumagne

[carns]

The older version of the function will not support the existing no_multi_recv or release_input_early fields, two options that we are not likely to use in the near term. So this isn't urgent, but we should take care of it for future proofing.