SignalHandler: fix infinite loop on BSD systems

SignalHandler::entry shouldn't poll for POLLOUT, because it never actually
writes to the pipes in question. Polling for POLLOUT causes poll(2) to
immediately return, so the function spins the CPU and never blocks. Remove
the POLLOUT flag, unnecessarily introduced in commit
8e4a78f169eda716c7d6811cb6db5c757dc67207 when switching from select() to poll().

This fixes the problem on FreeBSD and doesn't break anything (AFAICT) on
Linux. Tested on FreeBSD 9.1 amd64 and Ubuntu Server 13.04 amd64.

Fixes: #6492

Signed-off-by: Alan Somers <asomers@gmail.com>
Reviewed-by: Greg Farnum <greg@inktank.com>