Fix daemon issues caused by Ubuntu's surprising intermediary shell

Summary:
On OSX and other Linuxii, proc_open('./exec_daemon ...') opens a PHP process; on
Ubuntu it opens a "sh -c" process which opens a PHP process. The existence of
this surprising shell made everything stop working.

Use 'exec' to replace the shell with the PHP process.

Test Plan:
Ran daemons on OSX and Ubuntu, behavior seems okay in all cases.

Keep in mind I have absolutely no idea how Lunix works so this probably breaks
the world. (cc: simpkins)

Reviewed By: rm
Reviewers: rm, aran, tuomaspelkonen, jungejason
Commenters: aran
CC: simpkins, aran, epriestley, rm
Differential Revision: 212