Replace Latin abbreviation "cf.".

Replace Latin abbreviation "cf.".

People seem to be using "cf." ("confere"), which means "compare",
to mean "see" instead, for which the Latin abbreviation would be
"q.v." ("quod vide" -> "which see").

In some cases "cf." might actually be the correct term but it's
still not clear what specific aspects of a function/system call
one is supposed to be comparing.

I left one use in place in hope of obtaining clarification,
because it looks like it might be useful there, if contextualized.

Migrate these uses to English and add them to the list of
abbreviations to be avoided.

If the patch to vfork(2) is not accepted, then the cf. still needs
an \& after it because it is at the end of the line but not the
end of a sentence.

Signed-off-by: G. Branden Robinson <g.branden.robinson@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>