Make daemon overseers correctly emit signals in Ubuntu.

Summary:
Ubuntu's "kill" command does not support "--" to delimit PID/PGID arguments from
flags.

Test Plan:
^C'd an overseen daemon on ubuntu and OSX.

Reviewed By: rm
Reviewers: rm, tuomaspelkonen, jungejason, aran
CC: aran, rm, epriestley
Differential Revision: 324