Bug #382: Enhance davmail script to allow overrides

[SethRobertson]

It should be backwards compatible, but now you can pass in environmental variables (seen below) to override various default settings. Even better, if you just want to add java arguments (example: UI scaling) you can set JAVA_USER_OPTS and not have to worry about anything else.

Not fully tested since I don't have all of those environments.

CLASSPATH EXTRA_CLASSPATH JAVA JAVA_CORE_OPTS JAVA_EXTRA2_OPTS JAVA_EXTRA_OPTS JAVA_NET_OPTS_DFL JAVA_OPTS JAVA_USER_OPTS JFX_CLASSPATH LD_LIBRARY_PATH SWT_GTK3