libc and pthreads - Fix atfork issues with nmalloc, update dmalloc

libc and pthreads - Fix atfork issues with nmalloc, update dmalloc

* Implement atfork handling for nmalloc.  As part of this, refactor
  some of nmalloc.

* Remove ZERO_LENGTH_PTR from nmalloc.  Instead, force 0-byte
  allocations to allocate 1 byte.  The standard requires unique
  pointers to be returned.

* For now go back to a single depot lock instead of a per-zone
  lock.  It is unclear whether multi-threaded performance will
  suffer or not, but its the only way to implement atfork handling.

* Implement proper atfork interlocks for nmalloc via pthreads to avoid
  corruption when heavily threaded programs call fork().

* Bring dmalloc up to date in various ways, including properly
  implementing a minimum 16-byte alignment for allocations >= 16 bytes,
  and atfork handling.  Also use a global depot lock for the same
  reason we use it in nmalloc, and implement a front-end magazine
  shortcut for any allocations <= 2MB.

Reported-by: mneumann