As indicated by the XXX, this line of code was never meant to be
considered as "proper", mainly because it allocates a new parser
that is never freed for every xref section in the file.

The reason that this line was introduced in place of the commented-out
original above it is that the latter does not bounds-check the offset
but h_seek() does. It was a quick and reliable way to make invalid
offsets fail the parse.

Anyway, creating a new HParser *p that incidentally shadows the
previous occurance was meant to signal that this is just a temporary
name that we need real quick (on the next line), not a "proper" variable,
and that it probably wasn't there to stay. So with that sense in mind, I
am putting it back.

Remove -Wshadow from CFLAGS.

Moves program invocation details from README to pdf.1.mdoc.

Includes the generated ASCII output for convenience. Make sure to regenerate
with 'make doc' after changing the mdoc source.

TODO: -Wno flags from patch

This reverts commit fa8c763a.