Added the ability for a toolset to set a prefix for a target type, e.g., a
    "lib" prefix for a LIB type.  This was implemented by generalizing
    the existing capability to set a target file suffix.
    
      * build/type.jam
        (set-generated-target-suffix):  Refactored to forward call to new
        set-generated-target-ps rule.
        
        (change-generated-target-suffix):  Refactored to forward call to new
        change-generated-target-ps rule.
        
        (generated-target-suffix-real):  Renamed to generated-target-ps-real
        with argument indicating prefix or suffix.
        
        (generated-target-suffix):  Refactored to forward call to new
        generated-target-ps rule.
        
        (generated-target-ps):  New helper rule - returns assigned prefix or suffix
        acccording to prefix/suffix argument.
        
        (set-generated-target-prefix):  New rule sets target type prefix.
        
        (change-generated-target-prefix):  New rule changes target type prefix.
        
        (generated-target-prefix):  New rule returns target type prefix.
        
        (set-generated-target-ps):  New helper rule - sets prefix/suffix
	for a target type.
	
	(change-generated-target-ps):  New helper rule - changes
	prefix/suffix for a target type.
    
      * build/virtual-target.jam
        (add-prefix-and-suffix):  Invokes new type.generated-target-prefix
        to get assigned prefix.  Still some unfinished business with unix
        handling - suggest moving commented "hacky" stuff to unix toolset if that
        makes sense.
    
      * Formal test case not yet added.  Was tested with my custom hptns toolset.