RFR(M) 8209385: CDS runtime classpath checking is too strict when only classes from the system modules are archived

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8209385

webrev: http://cr.openjdk.java.net/~ccheung/8209385/webrev.00/

If a path such as -cp points to an non-empty directory is specified 
during CDS dump time but is not being referenced for class loading, we 
should not exit with the error "Cannot have non-empty directory in paths".

The fix is to skip checking the path entries which are not being 
referenced during CDS dump time. Adding the _max_used_path_index field 
in ClassLoaderExt so that the unreferenced path entries won't be checked 
during dump time. The _max_used_path_index will also be stored in the 
CDS archive header so that during run time, the unreferenced path 
entries will not be checked.

Testing: hs-tier 1,2,3.

thanks,
Calvin