[SPARK-9607] [SPARK-9608] fix zinc-port handling in build/mvn

- pass `$ZINC_PORT` to zinc status/shutdown commands
- fix path check that sets `$ZINC_INSTALL_FLAG`, which was incorrectly
  causing zinc to be shutdown and restarted every time (with mismatched
  ports on those commands to boot)
- pass `-DzincPort=${ZINC_PORT}` to maven, to use the correct zinc port
  when building

Author: Ryan Williams <ryan.blake.williams@gmail.com>

Closes #7944 from ryan-williams/zinc-status and squashes the following commits:

619c520 [Ryan Williams] fix zinc status/shutdown commands