PATCH: untaint method for IO::Handle, 5.003_06 version

PATCH: untaint method for IO::Handle, 5.003_06 version

This is a re-post of my patch to Graham's IO library to add a method in
IO::Handle called "untaint", that sets the IOf_UNTAINT flag on an object
that is of or inherits from IO::Handle. With this flag set, data read from
said handle is not tainted, whether running under -T, suid or sgid.

This patch adds the method to IO.xs, adds documentation and warning to the
pod of IO/Handle.pm, creates a new test in t/lib called io_taint.t, and
adds mention of the new file to MANIFEST.

Test suite for the untaint method of class IO::Handle.