Add editline support

GNU readline recently changed license to GPLv3+ which makes it
incompatible with chrony (GPLv2). This patch adds support for editline
library (BSD license).

client.c

@@ -41,9 +41,13 @@
 #include "memory.h"
 
 #ifdef FEAT_READLINE
+#ifdef USE_EDITLINE
+#include <editline/readline.h>
+#else
 #include <readline/readline.h>
 #include <readline/history.h>
 #endif
+#endif
 
 #ifdef HAS_STDINT_H
 #include <stdint.h>