Add the OpenJDK JRE, headless and jre_headless variants to the port
table. The new JAVA_RUN_FLAVOR knob picks the runtime variant. Ports
that build with Java keep using the JDK to build. JAVA_PORT set by the
user is now checked before use.

This does not change how any port is built or installed unless a user
sets one of these knobs. With neither set, the dependency metadata of
all USES=java ports is identical.

The knobs let users settle on one Java version for ports that support
several, so fewer JDKs get installed. They also let service ports such
as www/tomcat110 run on a headless JRE. That leaves the compiler,
debugger and attach tools, and the X11 libraries, off service hosts.

The UPDATING entry explains the details.

Approved by: java (maintainer)