Allow java.security to be extended via a properties directory

Currently, security properties are held within the java.security file in the JDK tree for each installed JDK. The system property java.security.properties can be used to point to a file containing additional properties. These can be appended to the existing set or override all existing properties.

There is currently no way to specify additional properties permanently or to reference multiple files. Making permanent changes to the java.security properties requires editing the java.security file in each JDK where the changes are required.

We can allow a directory tree to be specified either permanently in the java.security file by the security.propertiesDir property or on the command line using java.security.propertiesDir. Any property files found in this directory tree can be appended to those specified in java.security, as with the single file used by java.security.properties.

As an example, the security.propertiesDir in the java.security file of each JDK can be set to a common shared directory, allowing all JDKs to share a common set of security properties. This eases setting up properties on each new JDK installation and also allows the shared properties to be maintained under different access permissions to those of the JDK.

The command-line variant, java.security.propertiesDir, is intended primarily for testing and to disable a permanent properties directory by setting the value to empty. As with java.security.properties, the system property will be ignored if security.overridePropertiesFile in the java.security file is not set to true.