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).

m4/libevent.m4

@@ -1,33 +0,0 @@
-dnl Check to find the libevent headers/libraries
-
-AC_DEFUN([tinc_LIBEVENT],
-[
-  AC_ARG_WITH(libevent,
-    AS_HELP_STRING([--with-libevent=DIR], [libevent base directory, or:]),
-    [libevent="$withval"
-     CPPFLAGS="$CPPFLAGS -I$withval/include"
-     LDFLAGS="$LDFLAGS -L$withval/lib"]
-  )
-
-  AC_ARG_WITH(libevent-include,
-    AS_HELP_STRING([--with-libevent-include=DIR], [libevent headers directory]),
-    [libevent_include="$withval"
-     CPPFLAGS="$CPPFLAGS -I$withval"]
-  )
-
-  AC_ARG_WITH(libevent-lib,
-    AS_HELP_STRING([--with-libevent-lib=DIR], [libevent library directory]),
-    [libevent_lib="$withval"
-     LDFLAGS="$LDFLAGS -L$withval"]
-  )
-
-  AC_CHECK_HEADERS(event.h,
-    [],
-    [AC_MSG_ERROR("libevent header files not found."); break]
-  )
-
-  AC_CHECK_LIB(event, event_init,
-    [LIBS="-levent $LIBS"],
-    [AC_MSG_ERROR("libevent libraries not found.")]
-  )
-])