logmatcher: added store-matches warning to PCRE LogMatcher

As we are changing the default matcher implementation the warning on the
'store-matches' flag should be triggered by the PCRE matcher too. This
patch does exactly that.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
logmatcher: change the default matcher to PCRE instead of POSIX

This patch changes the default LogMatcher implementation starting
with syslog-ng 3.6. If the configuration indicates an earlier version
using the @version pragma, it still uses the POSIX implementation and
issues a warning to the user to update her config.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
LogProtoREMultiLineServer: use pcre instead of posix regexps

Now as PCRE is a hard dependency, we can prefer using that in our
multi-line implementation. It is faster anyway. The patch also
refactors multi-line regexps to a separate object.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>