unixify paths a bit more systematicaly in Pod-Html

unixify paths a bit more systematicaly in Pod-Html

Rather than unixifying paths sort of once off, we now do what I
threatened earlier: unixify all the --switch supplied paths during
command line parsing.

There are other bits of File::Spec being sprinkled around, quite
possibly in too-interior sections of the program, but this change
gets *all* tests passing on Win32 as well as GNU/Linux.

I think an audit of filepath normalization "border security" is
still due, but this may tide us over for now.