examples/csh: Case insensitive search in pager

This is one of the most commonly requested configurations I'm asked to
share or put in man(1). Currently I think this is the most appropriate
and cannonical place for it.

MFC after: 3 days
Reviewed by: carlavilla, imp
Approved by: carlavilla (mentor)
Differential Revision: https://reviews.freebsd.org/D49833

(cherry picked from commit ec95b6a3c77b72aca1823f1b118a7983c130e4df)