If dumpdev is AUTO but no dump device has been set -- i.e., there is no swap
space configured for rc.d/dumpon to designate for dumping -- then exit
silently rather than with a
> realpath: /dev/dumpdev: No such file or directory
error message.

An argument could be made that we should print a (more informative) warning
message; but given that under the same conditions the rc.d/dumpon script will
already print a
> No suitable dump device was found
warning, it seems that printing an additional
> Dump device does not exist.  Savecore not run.
warning would be superfluous.
Fix typo in previous commit: Exit if */dev/dumpdev* does not exist, not if
*/bin/realpath* does not exist...

Submitted by:	markj
Pointy hat to:	cperciva