Add a new interceptor for getvfsstat(2) from NetBSD

Summary:
getvfsstat - gets list of all mounted file systems.

Add a dedicated test.

Reviewers: vitalybuka, joerg

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, mgorny, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D55014

llvm-svn: 348027