dumpon(8): Make error messages clearer

dumpon(8): Make error messages clearer

Before this patch, when a dump device is already configured, dumpon(8)
only complains with 'Device busy', which isn't helpful.  Now, dumpon(8)
will tell whether the dump device is already configured or whether need
to run 'dumpoff' first.

Always show verbose information.  The utility still accepts the '-v'
option for backward compatibility, but don't mention the option in the
man page anymore.

Bugs: https://bugs.dragonflybsd.org/issues/3092