logger: refactor long if clause

When if clause that continues throughout whole function it usually can be
shorten to immediate action, e.g., in this case return on the spot not at
end of the function.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
docs: make logger.1 facilities and levels easier to read

Most people read vertical list lot quicker than horizontal.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
logger: add function pointer to choose how logging is done

This change paves way to adding support for both RFC 3164 and RFC 5424
syslog protocols.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>