capture: add sendfile() fallback
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.
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