Load java.conf from all prefixes
Don't nullify JAVACMD_OPTS by default

If low-precedence java.conf sets come Java options then they shouldn't
be reset by higher-precedence config file.
Rewrite _set_java_home() to support multiple prefixes
Cleanup set_jvm() function
Cleanup set_classpath() function
Cleanup set_javacmd() function
Simplify set_flags() and set_options() functions
Cleanup run() function
Rewrite set_jvm_dirs() to support multiple prefixes
Rewrite find_jar() to use new directory lists
Set defaults for _javadirs and _jvmdirs

These defaults mey be useful in case java.conf is absent.
Cleanup check_java_env() function