[perl #57512] Warnings for implicitly closed handles

[perl #57512] Warnings for implicitly closed handles

If the implicit close() fails, warn about it, mentioning $! in the
message.  This is a default warning in the io category.

We do this in two spots, sv_clear and gp_free.  While sv_clear would
be sufficient to get the warning emitted, the warning won’t contain
the name of the handle when called from there, because lone IO thing-
ies are nameless.  Doing it also when a GV’s glob pointer is freed--as
long as the IO thingy in there has a reference count of 1--allows the
name to be included in the message, because we still have the glob,
which is where the name is stored.

The result:

$ ./miniperl -Ilib -e 'open fh, ">/Volumes/Disk Image/foo"; print fh "x"x1000, "\n" for 1..50; undef *fh'
Warning: unable to close filehandle fh properly: No space left on device at -e line 1.