degesch: use libffi to unify input callbacks

And fuck you both, Readline and Editline.

README.adoc

@@ -10,7 +10,7 @@ All of them have these potentially interesting properties:
 
  - full IPv6 support
  - TLS support, including client certificates
- - minimal dependencies
+ - lean on dependencies (with the exception of 'degesch')
  - compact and arguably easy to hack on
  - permissive license
 
@@ -66,9 +66,9 @@ support (even though socksify can add that easily to any program).
 Building
 --------
 Build dependencies: CMake, pkg-config, help2man, awk, sh, liberty (included) +
-Runtime dependencies: openssl, curses (degesch),
-                      readline >= 6.0 or libedit >= 2013-07-12 (degesch),
-                      lua >= 5.3 (degesch, optional)
+Runtime dependencies: openssl +
+Additionally for degesch: curses, libffi, lua >= 5.3 (optional),
+                          readline >= 6.0 or libedit >= 2013-07-12
 
  $ git clone --recursive https://github.com/pjanouch/uirc3.git
  $ mkdir uirc3/build