better implementation of change#3326; open(local $foo,...) now

better implementation of change#3326; open(local $foo,...) now
allowed in addition to any uninitialized variable, for consistency
with how autovivification works elsewhere; add code to use the
variable name as the name of the handle for simple variables, so
that diagnostics report the handle: "... at - line 1, <$foo> line 10."

p4raw-link: @3326 on //depot/perl: 853846ea710f8feaed8c98b358bdc8967dd522d2

p4raw-id: //depot/perl@4639