logger: warn when --file and command line message are combined

logger: warn when --file and command line message are combined

When --file is combined with command line arguments the later has
silently been ignored.  This commit makes user to be aware the logger
will not use command line arguments when --file is specified.

Reported-by: "Daniel 'DaB.' Baur" <debian@daniel.baur4.info>
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=467244
CC: Andreas Henriksson <andreas@fatal.se>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>