Improve POSIX compliance by making case-insensitive extended

Improve POSIX compliance by making case-insensitive extended
regular expressions the default in man(1) -k searches, also matching
what the man-db package used by many Linux distributions does.

Originally requested by Wolfram Schneider <wosch at FreeBSD>
via Yuri Pankov <yuripv at FreeBSD>.

Feedback and OK cheloha@, and no objections when shown on tech@.

Thanks to cheloha@ for pointing out that POSIX requires this behaviour
and for the suggestion to explicitly say that *extended* regular
expressions are used here.

While here, unify spelling of case-[in]sensitive, fix a typo,
update the EXAMPLES, and add a STANDARDS section.