zlib: Make the finish flush flag configurable

Up to now, `Z_FINISH` was always the flushing flag that was used
for the last chunk of input data. This patch makes this choice
configurable so that advanced users can perform e.g. decompression of
partial data using `Z_SYNC_FLUSH`, if that suits their needs.

Add tests to make sure that an error is thrown upon encountering
invalid `flush` or `finishFlush` flags.

Fixes: https://github.com/nodejs/node/issues/5761
PR-URL: https://github.com/nodejs/node/pull/6069


Reviewed-By: James M Snell <jasnell@gmail.com>