Use std::vector in event-loop.cc

In my occasional and continuing campaign against realloc, this patch
changes event-loop.cc to use std::vector to keep track of pollfd
objects.  Regression tested on x86-64 Fedora 38.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Approved-By: John Baldwin <jhb@FreeBSD.org>