Create autotools plumbing for SoftEther.

Add autoconf and automake capabilities to SoftEther so that it can be built
like this:

	# autoreconf --force --install
	# ./configure
	# make install DESTDIR=/tmp/softether

All standard configure parameters are supported, plus:

	# ./configure --enable-debug

Autotools support makes porting, cross compiling, and optimization much easier.

These GNU autoconf-archive components are used for dependency checking:

  * ax_check_openssl.m4
  * ax_check_zlib.m4
  * ax_lib_readline.m4
  * ax_pthread.m4
  * ax_with_curses.m4

NB: http://www.gnu.org/software/autoconf-archive/
Create libsoftether.so and dynamically link the userland.

Sharing object code between vpnbridge, vpnclient, vpnserver, and vpncmd
reduces the binary size of SoftEther by 85% and its administrative memory
footprint by 50%.
Use FHS installation directories.

Install to `/usr/sbin`, `/usr/lib`, and `/var/lib` according to the Linux
filesystem hierarchy standard if SoftEther is built through autotools.

In a managed installation, the FHS stipulates that the application must
accomodate a read-only installation path.  This requires a new `GetStateDir`
function that substitues `GetExeDir` in some parts of the code.
Create a non-forking softetherd for upstart and systemd.

Implement a daemon that expects to be invoked by a new-style init like upstart
or systemd as:

	/usr/sbin/softetherd [vpnbridge|vpnclient|vpnserver]

Alternatively, if the command line argument is empty, then use the
`SOFTETHER_MODE` environment variable instead.