ping: Specify the sigaction(2) name

After a2a008880568eaeb87d8404b39627b83df851f34, specify the signal name.

Obtained from: Darwin
MFC after: 1 week
Reviewed by: asomers
Pull Request: https://github.com/freebsd/freebsd-src/pull/863
Differential Revision: https://reviews.freebsd.org/D38484