fix PathTools taint handling for Perl 5.6

fix PathTools taint handling for Perl 5.6

PathTools's reserve code for detecting tainted values on Perl 5.6
generated a warning if the value was undefined, which it commonly was.
Make it allow all undefined values (which get filtered out in the next
step) without warning.  Its test script for tainting behaviour also failed
to detect whether tainting was turned on on Perl 5.6, incorrectly skipping
the test script on the basis that the Perl doesn't support tainting.
Switch that check to an empirical arrangement that works on any Perl.