Avoid perldoc UID problems *and* shell redirection problems with a utility script

See also Pull #973 and issue #520

The shell redirection trick is no longer working for us downstream anymore, so we've applied this patch downstream to get it working. 

It changes everywhere that invokes `perldoc -u` to invoke `podextract.pl` instead.

The only thing I don't really understand is the check in `perldoc.pm`, and that name should possibly be modified, and the check for Perldoc should possibly be different.

e.g:

```perl
capture_output('perldoc -ud foo bar')
```

May no longer be appropriate, or required.

```perl

eval { require Pod::Perldoc::ToPod } and Pod::Perldoc::ToPod->can('parse_from_file')
```

May be appropriate instead.

( ie: If we fix this call of perldoc, we can rule out the narrow risk of perldoc accidentally failing in configure due to UID=0 → UID=nobody )