serconf: make the command-line parsing more flexible

The tty device can be anywhere is the arguments list, instead of
the first one only.
serconf: exit if no device is found
serconf: apply settings only if there are settings

If there are no settings after parsing the command-line,
do not reconfigure the tty.
ser{chrono,term}: always call serconf

Even if there is only one parameter.
serconf: rework the setting parser

Make a better use of sed and avoid read.