sys: Remove/update references to the swapper process in various comments

[olce]

There is an occurrence of "swapper" in sys/vm/vnode_pager.c that seems to also point to the swapper process.

[jhb]

In D54051#1234938, @olce wrote:

There is an occurrence of "swapper" in sys/vm/vnode_pager.c that seems to also point to the swapper process.

I think that might be a reference to the swap pager as swap_pager_putpages() can do asynchronous I/O. I probably should fix that comment though to reference the swap pager now that I've looked at it in more detail though. There are a couple of "swapper" mentions in swap_pager.c that are clearly about the swap pager, as well, but they are not as trivial to rewrite.