Add interceptors for the fts(3) API family from NetBSD

Summary:
fts(3) is API to traverse a file hierarchy.
Cover this interface with interceptors.

Add a test to validate the interface reading
the number of regular files in /etc.

Based on original work 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/D54247

llvm-svn: 348227