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. Ins

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

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