ionice: add strtol() checks, cleanup usage text and man page

ionice: add strtol() checks, cleanup usage text and man page

 * cleanup usage() output

 * check strtol(); don't ignore wrong command line options

The original ionice design was a little broken, because it was
possible to specify a PID and also a COMMAND:

	ionice -c2 -p 123 /bin/foo

but the command /bin/foo was executed without requested scheduling
class. That's stupid behaviour.

Now you have to use "-p PID" **or** COMMAND, but not both. Nothing is
ignored and all options are checked.

Signed-off-by: Karel Zak <kzak@redhat.com>