We want the capture format to be usable on systems other than
Linux so that it can be used in places like GTK. To do this,
we need a sendfile() fallback that can be portable/useful on
those systems.
FreeBSD has sendfile(), but the semantics differ slightly. This
implementation will walk up to the nearest page size and then
do page aligned pread()/write() which is likely good enough of
an implementation anyway.