Implemented `UnixBufferQueue::SetCapacity` so that `setsockopt`
with `SO_RCVBUF` will not always return an error.
Change-Id: I2d9be84633f84474fac64b379e9f89ef2751a094
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6816
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>