Retry FS system calls on EINTR

Summary: EINTR means 'please retry'. We don't do that currenty. We should.

Test Plan: make check, although it doesn't really test the new code. we'll just have to believe in the code!

Reviewers: haobo, ljin

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17349