Fix util/mkpod2html.pl to call pod2html with absolute paths

Fix util/mkpod2html.pl to call pod2html with absolute paths

It turns out that on VMS, pod2html only recognises VMS directory
specifications if they contain a device name, which is accomplished by
making them absolute.  Otherwise, a VMS build that includes building
the document HTML files ends up with an error like this:

    $ perl [---.downloads.openssl-3_0-snap-20210916.util]mkpod2html.pl -i [---.downloads.openssl-3_0-snap-20210916.doc.man1]CA.pl.pod -o [.DOC.HTML.MAN1]CA.PL.HTML -t "CA.pl" -r "[---.downloads.openssl-3_0-snap-20210916.doc]"
    [---.downloads.openssl-3_0-snap-20210916.util]mkpod2html.pl: error changing to directory -/-/-/downloads/openssl-3_0-snap-20210916/doc/: no such file or directory
    %SYSTEM-F-ABORT, abort

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16626)