savecore: accept device names without the /dev/ prefix

[asomers]

Summary

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
Sponsored by: Axcient

[cem]

Just style/typo nits. Functionally, it looks good.

sbin/savecore/savecore.c

1004

Should this be else if or on a separate line?

1175–1177

Seems weird to add a single-line {} here

[bcr]

Also good by manpages. Thanks for this change!