Call FETCH once on handle passed as 3rd arg to open()

Call FETCH once on handle passed as 3rd arg to open()

When open() has three arguments and the second ends with & the third
argument is treated as a handle.

In some cases get-magic was being skipped; in others, it was being
called three times.

This commit fixes it by modifying sv_2io.

In 5.8.x (before commit 7a5fd60d4), sv_2io did not call get-magic at
all except when croaking ("Bad filehandle: %"SVf).  In 5.10.0 (after
commit 7a5fd60d4), it started calling get-magic only if the sv was
neither a glob, a reference, or undef.  So it has never been reliable
in its invocation of get-magic.

sv_2io now consistently skips get-magic on the sv passed in directly
to it (open(), the only caller in the core, has already called get-
magic before passing it in).  It now calls get-magic on SvRV(sv) if
what is passed in is a reference, so open(fh, ">&", \$tied) will work.

Interestingly, open supports multiple levels of references:

\\\\\\\\\\\\open+f,">&",\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
 \\\\\\\\\\\*STDOUT;print{f}"Just another Perl hacker,\n",.\\\\\\\y\\\