perlxs: add some links and change formatting codes for non-files

[Grinnz]

Added a few links to utilities and modules that were not previously linked. Added F<> on a few instances of Makefile.

The F<> formatting code generally is used for filenames; to distinguish instances of unlinked utilities and modules from actual and hypothetical filenames such as Makefile, I replaced them with B<>. However, I don't have a strong preference on which code is used, or even if it is preferred to leave them as F<>. B<> and C<> are most commonly used for this purpose, I<> is closer to how F<> renders but is asterisked when rendered to plain text. Here are how each code usually renders in the 3 main ways perldoc is consumed:

• B - displays bold in HTML and manpage, no formatting in plain text
• C - displays as code in HTML, quoted in manpage and plain text
• I - displays italicized in HTML, underlined in manpage, asterisked in plain text
• F - displays italicized in HTML, underlined in manpage, no formatting in plain text
• L - link in HTML, no formatting in manpage or plain text

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• This set of changes does not require a perldelta entry.

[khwilliamson]

I think it is better to use C<> than B<> for subsequent references to xsubpp, and for the L<> reference to also add C<>, but I'm not certain. I'd go with whatever the consensus ends up; otherwise lgtm