Created a lightweight abstraction layer so that another i/o
reactor can be dropped in place of asio.
The basic approach is to rename all references to asio::xxx
types to openvpn_io::xxx and then make openvpn_io a
preprocessor variable that points to the top-level namespace
of the i/o reactor implementation.
All of the source files that currently include <asio.hpp> now
include <openvpn/io/io.hpp> instead:
This gives us a lightweight abstraction layer that allows us
to define openvpn_io to be something other than asio.
Other changes:
* Inclusion of asio by scripts/build is now optional, and is
enabled by passing ASIO=1 or ASIO_DIR=<dir>.
* Refactored openvpn/common/socktypes.hpp to no longer
require asio.
* Refactored openvpn/log/logthread.hpp to no longer require
asio.
* Added openvpn::get_hostname() method as alternative to
calling asio directly.
* openvpn/openssl/util/init.hpp will now #error
if USE_ASIO is undefined.
Signed-off-by: James Yonan <james@openvpn.net>
