Support preferentially the Unicode 'scripts' definition

Support preferentially the Unicode 'scripts' definition
in the \p{In...} notation since according to Unicode the
scripts concept is more natural for matching than using
the somewhat artificial block names.  The block names are
still available, though, and if there's a name conflict,
the scripts one wins and the blocks one has to do with
'Block' appended to its name.  For more information see

http://www.unicode.org/unicode/reports/tr24/

p4raw-id: //depot/perl@11132