eal: introduce internal wrappers for file operations

eal: introduce internal wrappers for file operations

Introduce OS-independent wrappers in order to support common EAL code
on Unix and Windows:

* eal_file_open: open or create a file.
* eal_file_lock: lock or unlock an open file.
* eal_file_truncate: enforce a given size for an open file.

Implementation for Linux and FreeBSD is placed in "unix" subdirectory,
which is intended for common code between the two. These thin wrappers
require no special maintenance.

Common code supporting multi-process doesn't use the new wrappers,
because it is inherently Unix-specific and would impose excessive
requirements on the wrappers.

Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>