Allow the start and stop ports, and additional Java options for the standalone Jetty package to be specified using environment variables

This is already done for the XWIKI_OPTS variable on non-Windows, but it should also apply for JETTY_PORT and JETTY_STOP_PORT, both in the .bat and .sh scripts.

Also, JETTY_STOP_PORT is inconsistently named JETTY_STOPPORT in the .sh scripts.