Stop seek($glob_copy...) from clearing PL_last_in_gv

Stop seek($glob_copy...) from clearing PL_last_in_gv

seek had the same bug as tell.  Here is the commit message from
8dc99089, which fixed tell:

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Stop tell($glob_copy) from clearing PL_last_in_gv

This bug is a side effect of rv2gv’s starting to return an incoercible
mortal copy of a coercible glob in 5.14:

$ perl5.12.4 -le 'open FH, "t/test.pl"; $fh=*FH; tell $fh; print tell'
0
$ perl5.14.0 -le 'open FH, "t/test.pl"; $fh=*FH; tell $fh; print tell'
-1

In the first case, tell without arguments is returning the position of
the filehandle.

In the second case, tell with an explicit argument that happens to
be a coercible glob (tell has an implicit rv2gv, so tell $fh is actu-
ally tell *$fh) sets PL_last_in_gv to a mortal copy thereof, which is
freed at the end of the statement, setting PL_last_in_gv to null.  So
there is no ‘last used’ handle by the time we get to the tell without
arguments.

This commit adds a new rv2gv flag that tells it not to copy the glob.

By doing it unconditionally on the kidop, this allows tell(*$fh) to
work the same way.

Let’s hope nobody does tell(*{*$fh}), which will unset PL_last_in_gv
because the inner * returns a mortal copy.

This whole area is really icky.  PL_last_in_gv should be refcounted,
but that would cause handles to leak out of scope, breaking programs
that rely on the auto-closing ‘feature’.