savecore: accept device names without the /dev/ prefix

dumpon has accepted device names without the prefix ever since r291207.
Since dumpon and savecore are always paired, they ought to accept the same
arguments. Prior to this change, specifying 'dumpdev="da3"' in
/etc/rc.conf, for example, would result in dumpon working just fine but
savecore complaining that "Dump device does not exist".

PR: 247618
Reviewed by: cem, bcr
MFC after: 2 weeks
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D25500