allow mapping null page

+ User has to 'free' the 0 page before it becomes accessible to mapping.
  Probably worth noting that this is likely VERY niche and mainly
  concerns stuff like certain kinds of emulators that I'm still
  eons from implementing, but still.

  Unbacked pages can also be useful for some kinds of notifications,
  like 'if someone writes to this page, please report it to me with this
  ID' or whatever, I remember seeing some discussion about it somewhere
  but that's also not really relevant for the moment.

  Most significantly, at least with the current design,
  after the null page is freed it becomes available for use to regular req_mem()
  calls, so users should probably using locks around memory requests.
  That's probably a good idea anyway as internally the kernal has to
  lock the virtual memory, and without a scheduler it might cause
  threads to spin for a while in the kernel which is rather bad.

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