Drop libevent and use our own event handling again.

There are several reasons for this:

- MacOS/X doesn't support polling the tap device using kqueue, requiring a
  workaround to fall back to select().
- On Windows only sockets are properly handled, therefore tinc uses a second
  thread that does a blocking ReadFile() on the TAP-Win32/64 device. However,
  this does not mix well with libevent.
- Libevent, event just the core, is quite large, and although it is easy to get
  and install on many platforms, it can be a burden.
- Libev is more lightweight and seems technically superior, but it doesn't
  abstract away all the platform differences (for example, async events are not
  supported on Windows).

doc/tinc.texi

@@ -339,7 +339,6 @@ having them installed, configure will give you an error message, and stop.
 * OpenSSL::
 * zlib::
 * lzo::
-* libevent::
 * libcurses::
 * libreadline::
 @end menu
@@ -458,27 +457,6 @@ make sure you build development and runtime libraries (which is the
 default).
 
 
-@c ==================================================================
-@node       libevent
-@subsection libevent
-
-@cindex libevent
-For the main event loop, tinc uses the libevent library.
-
-If this library is not installed, you wil get an error when configuring
-tinc for build.
-
-You can use your operating system's package manager to install this if
-available.  Make sure you install the development AND runtime versions
-of this package.
-
-If you have to install libevent manually, you can get the source code
-from @url{http://libevent.org/}. Instructions on how to configure,
-build and install this package are included within the package.  Please
-make sure you build development and runtime libraries (which is the
-default).
-
-
 @c ==================================================================
 @node       libcurses
 @subsection libcurses