Add new interceptor for strtonum(3)

Summary:
strtonum(3) reliably convertss string value to an integer.
This function is used in OpenBSD compat namespace
and is located inside NetBSD's libc.

Add a dedicated test for this interface.

It's a reworked version of the original code by Yang Zheng.

Reviewers: joerg, vitalybuka

Reviewed By: vitalybuka

Subscribers: tomsun.0.7, kubamracek, llvm-commits, mgorny, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D54527

llvm-svn: 348651