Correct the reason for some PERL5LIB reset code.
Ensure PERL5LIB is always propagated into @INC even for -T tests.

It translates PERL5LIB into -I switches if taint mode is on.

Test::Harness already does this, but TAP::Harness and prove do not.
The Test::Harness code may now be redundant.

This makes local::lib and similar environments which depend on PERL5LIB
for regular operation work.

See rt.cpan.org 84377.
Merge pull request #11 from Perl-Toolchain-Gang/PERL5LIB_and_taint

PERL5LIB and taint