Add new case insensitive match capability.

Add boolean flag to pkg_is_installed() to allow case insensitve
matching there.

Use pkg_is_installed() instead of duplicating the code in several
places.
Fix typo: the option for 'exact match' is -e
Revise pkg-set.8 to use better English.  Sort option descriptions
alphabetically, add description of '-e' for exact match.
Add '-e' option to pkg-set.  Sort options. Delete unused 'k' from
getopts() string.  Make handling of 'yes' more like other programs.
On second thoughts: '-e' for exact search is pointless, as exact is
already the default.
Think again.  Add a global case_sensitive flag which defaults to true
(case sensitive), plus accessor and mutator functions.

Adjust MATCH_EXACT and MATCH_REGEX code to abide by the case
sensitivity flag.
Generally add a -i option meaning 'case insensitive' which affects
MATCH_EXACT and MATCH_REGEX match types.