Summary:

Add close_fd_mask functionality to AFL driver.

Summary:
Add support for env var AFL_DRIVER_CLOSE_FD_MASK which behaves
the same as libFuzzer's -close_fd_mask=1.

Also add tests.

Reviewers: kcc, vitalybuka, morehouse

Reviewed By: morehouse

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D60334

llvm-svn: 358703