[scudo] Zero- and pattern-initialization of memory.

[scudo] Zero- and pattern-initialization of memory.

Summary:
Implement pattern initialization of memory (excluding the secondary
allocator because it already has predictable memory contents).
Expose both zero and pattern initialization through the C API.

Reviewers: pcc, cryptoad

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D79133