journalctl: make matching optionally case sensitive

Case sensitive or case insensitive matching can be requested using
--case-sensitive[=yes|no].

Unless specified, matching is case sensitive if the pattern contains any
uppercase letters, and case insensitive otherwise. This matches what
forward-search does in emacs, and recently also --ignore-case in less.  This
works surprisingly well, because usually when one is wants to do case-sensitive
matching, the pattern is usually camel-cased. In the less frequent case when
case-sensitive matching is required with an all-lowercase pattern,
--case-sensitive can be used to override the automatic logic.