External Public-Key Authentication API (#72) * Implemented dynamic loading of an external plug-in shared library to delegate public key authentication * Moved conditional compilation of the plugin infrastructure into the configure.ac script to be able to add -ldl to dropbear build only when the flag is enabled * Added tags file to the ignore list * Updated API to have the constructor to return function pointers in the pliugin instance. Added support for passing user name to the checkpubkey function. Added options to the session returned by the plugin and have dropbear to parse and process them * Added -rdynamic to the linker flags when EPKA is enabled * Changed the API to pass a previously created session to the checkPubKey function (created during preauth) * Added documentation to the API * Added parameter addrstring to plugin creation function * Modified the API to retrieve the auth options. Instead of having them as field of the EPKASession struct, they are stored internally (plugin-dependent) in the plugin/session and retrieved through a pointer to a function (in the session) * Changed option string to be a simple char * instead of unsigned char *

--- a/session.h	Wed Mar 27 22:15:23 2019 +0800
+++ b/session.h	Wed May 15 09:43:57 2019 -0400
@@ -38,6 +38,9 @@
 #include "chansession.h"
 #include "dbutil.h"
 #include "netio.h"
+#if DROPBEAR_EPKA
+#include "pubkeyapi.h"
+#endif
 
 void common_session_init(int sock_in, int sock_out);
 void session_loop(void(*loophandler)(void)) ATTRIB_NORETURN;
@@ -216,6 +219,10 @@
 	volatile int exitflag;
 	/* set once the ses structure (and cli_ses/svr_ses) have been populated to their initial state */
 	int init_done;
+
+#if DROPBEAR_EPKA
+        struct EPKASession * epka_session;
+#endif
 };
 
 struct serversession {
@@ -241,6 +248,14 @@
 	pid_t server_pid;
 #endif
 
+#if DROPBEAR_EPKA
+        /* The shared library handle */
+        void *epka_plugin_handle;
+
+        /* The instance created by the plugin_new function */
+        struct EPKAInstance *epka_instance;
+#endif
+
 };
 
 typedef enum {