[scudo] Simplify initialization and flags

Summary:
This is mostly some cleanup and shouldn't affect functionalities.

Reviewing some code for a future addition, I realized that the complexity of
the initialization path was unnecessary, and so was maintaining a structure
for the allocator options throughout the initialization.

So we get rid of that structure, of an extraneous level of nesting for the
`init` function, and correct a couple of related code inaccuracies in the
flags cpp.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39974

llvm-svn: 318157