Allow to configure the used parser for a monitored file.

Allow to configure the used parser for a monitored file.

This commit adds support to assign the used parser for a
configured monitored file. The information which parser
should be used directly will be obtained from the
configuration option.

For example "Monitor_Snort = /path/to/snort/alert/file" will
monitor the specified file as before, but will try to use
the parser called "snort" (parsers internal completely are handled
in lower case format) to parser any messages which are written to
that file.

This will remove the static filename <-> parser structure, which will
allow a better implementation of parsers or customized parsers in the
future and a better portability to other systems.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>