Summary: include/posix/io_posix.h doesn't not prevent multiple includes. Need to fix it. It is also breaking unity build.
Test Plan: Run unity build and see error go away.
Reviewers: rven, igor, IslamAbdelRahman, kradhakrishnan, anthony
Subscribers: leveldb, dhruba
Differential Revision: https://reviews.facebook.net/D49281
Summary: This patch splits the posix storage backend into Env and
the actual *File implementations. The motivation is to allow other Envs
to use posix as a library. This enables a storage backend different from
posix to split its secondary storage between a normal file system
partition managed by posix, and it own media.
Test Plan: No new functionality is added to posix Env or the library,
thus the current tests should suffice.
