Update the ESMC_Finalize interface to match ESMC_Initialize and add endFlag

[billsacks]

This emerges from @oehmke 's comment in PR 234: We want to update the ESMC_Finalize interface to more closely match the ESMC_Initialize interface (e.g., adding *rc and having multiple argument capability). In addition – and this is the connection to #234 – we want to add the new endFlag argument as an optional argument, at which point the new ESMC_FinalizeWithFlag can be removed.

Note that this will involve changing a public interface, which is why we didn't do it with #234.

When making this change, consider bringing the test program from #234 (comment) in as a unit test - or at least running it manually to verify that it still works as intended.