savecore: emit information about saved cores under verbose

Declare how many cores we saved, and where we saved them to. Drop a
comment about emitting little information; it's obvious from the block
in question that we emit nothing without verbose.

Reviewed by: markj
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
X-NetApp-PR: #65
Differential Revision: https://reviews.freebsd.org/D31393