mainly to improve how the web versions render

... by using the "\fIopt(3)\fP" syntax they will be linked properly when
the web version of the page is generated.

c-ares now does support IPv6;
avoid implying threaded resolver is Windows-only;
two referenced source files were renamed in 7de2f927

... to the always-copy-char *-argument.

And fix some minor mistakes.

With all the new individual option man pages created, this now refers to
each separate one instead of duplicaing the info. Also makes this page
easier to overview.

shorten descriptions, mostly refer to the separate descriptions

... and fixed mancheck to ignore obsolete options

Now all current options have their own man pages.

To be further discussed, debated and edited

This is consistent with the existing obsolete error code naming
convention.