POM bug: separate jvm.arg.allowSecurityManager from jvm.arg.addOpens

POM bug: separate jvm.arg.allowSecurityManager from jvm.arg.addOpens

Before, this was in a single variable, and for JDK 18+, the security
manager setting also falsely overwrote the '--add-opens' command. This
was the root cause for a few dozen LTW tests to fail on JDK 18, if they
were not run in full LTW mode, i.e. in a separate JVM.

After this fix, it should be possible to revert the corresponding
commits, at least their non-cosmetic parts.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>