Send an explicit log message from daemons when they receive a signal

Summary: Ref T3557. Currently, we print a message to stdout with `echo` but don't actually log it. Instead, log it explicitly. This still prints it, but timestamps it, shows signal information, and gets it into the log. Notably, this helps distinguish between standard healthy exits (responding to SIGINT) and less-healthy ones.

Test Plan: {F51255}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, kartikvishwanath

Maniphest Tasks: T3557

Differential Revision: https://secure.phabricator.com/D6545