Make perlbug encoding-agnostic in handling prepared reports.

Guessing the encoding of a report file prepared ahead of time is
really, well, too much guesswork, plus there are at least a couple
of mistakes in the implementation.  And we weren't even trying to
guess for reports created on-the-fly in an editor, which is a bit
inconsistent.

So handle prepared reports the same way as on-the-fly reports and
attachments, which means all I/O is done using the :raw layer.

This is only "Unicode-aware" in the sense that we're aware there
are a lot of encodings out there and we're trying not to mangle
them in transit by accidental conversion.  We're not doing any
explicit character set conversions and we shouldn't assert in the
MIME headers that we know what character set we're sending because
we don't.
Attempt to satisfy CRLF expectations in perlbug on Windows.
initialise PL_watch_pvx

This global var is only used in debugging builds, but with
PERL_GLOBAL_STRUCT_PRIVATE builds, it gives valgrind errors. Just
initialise it to NULL.

I was originally trying to replicate smoke failures of
dist/Thread-Queue/t/07_lock.t under PERL_GLOBAL_STRUCT_PRIVATE, and
valgrind complained.