cmp: Print a summary on SIGINFO.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D38280

(cherry picked from commit 6673a5476d029cd5b47b2eed27032211a14f52bd)

cmp: Increase buffer size for non-mmap case.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: rpokala
Differential Revision: https://reviews.freebsd.org/D38281

(cherry picked from commit 134841a7f3e5516a83732d40a4d42268c901a492)