Have tincctl act as a shell when no command is given.

By default it uses readline to read commands. If the input and output are not a
tty, no prompt is shown.

m4/readline.m4

@@ -0,0 +1,41 @@
+dnl Check to find the readline headers/libraries
+
+AC_DEFUN([tinc_READLINE],
+[
+  AC_ARG_ENABLE([readline],
+    AS_HELP_STRING([--disable-readline], [disable readline support]))
+  AS_IF([test "x$enable_readline" != "xno"], [
+  AC_DEFINE(HAVE_READLINE, 1, [have readline support])
+    readline=true
+    AC_ARG_WITH(readline,
+      AS_HELP_STRING([--with-readline=DIR], [readline base directory, or:]),
+      [readline="$withval"
+       CPPFLAGS="$CPPFLAGS -I$withval/include"
+       LDFLAGS="$LDFLAGS -L$withval/lib"]
+    )
+
+    AC_ARG_WITH(readline-include,
+      AS_HELP_STRING([--with-readline-include=DIR], [readline headers directory]),
+      [readline_include="$withval"
+       CPPFLAGS="$CPPFLAGS -I$withval"]
+    )
+
+    AC_ARG_WITH(readline-lib,
+      AS_HELP_STRING([--with-readline-lib=DIR], [readline library directory]),
+      [readline_lib="$withval"
+       LDFLAGS="$LDFLAGS -L$withval"]
+    )
+
+    AC_CHECK_HEADERS([readline/readline.h readline/history.h],
+      [],
+      [AC_MSG_ERROR("readline header files not found."); break]
+    )
+
+    AC_CHECK_LIB(readline, readline,
+      [READLINE_LIBS="-lreadline"],
+      [AC_MSG_ERROR("readline library not found.")]
+    )
+  ])
+
+  AC_SUBST(READLINE_LIBS)
+])