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

[esabol]

This is a good idea, @SethRobertson ! May I suggest running the script through shellcheck (see https://www.shellcheck.net/)? It's an excellent linter for shell scripts. For example, it suggests using

: "${JAVA_CORE_OPTS:=-Xmx512M -Dsun.net.inetaddr.ttl=60}"

instead of

: ${JAVA_CORE_OPTS:="-Xmx512M -Dsun.net.inetaddr.ttl=60"}

[mguessan]

Launch script changed quite a lot, merged the "override JAVA_OPTS" idea based on just 3 variables $BASE_JAVA_OPTS $JFX_JAVA_OPTS $SWT_JAVA_OPTS

Please test this to confirm it's working in various environments.