Fix bugs introduced with the amap rework

Fix bugs introduced with the amap rework

- The number of slots must be initialized in the chunk of a small amap,
  otherwise unmapping() part of a mmap()'d range would delay freeing
  of vm_anons for small amaps
- If the first chunk of a bucket is freed, check if the next chunk in
  the list has to become the new first chunk
- Use a separate loop for each type of traversal (small amap, by bucket
  by list) in amap_wiperange(). This makes the code easier to follow and
  also fixes a bug where too many chunks were wiped out when traversing
  by list

However, the last two bugs should happen only when turning a previously
private mapping into a shared one, then forking, and then having
both processes unmap a part of the mapping.

snap and ports build tested by krw@, review by kettenis@