perl: ensure File::Spec::canonpath() preserves taint [CVE-2015-8607]

perl: ensure File::Spec::canonpath() preserves taint [CVE-2015-8607]

Beginning in PathTools 3.47 and/or perl 5.20.0, the File::Spec::canonpath()
routine returned untained strings even if passed tainted input. This defect
undermines the guarantee of taint propagation, which is sometimes used to
ensure that unvalidated user input does not reach sensitive code.

This defect was found and reported by David Golden of MongoDB, and a patch
was provided by Tony Cook.

References:

 * https://rt.perl.org/Public/Bug/Display.html?id=126862
 * https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-8607

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>