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 *

-----BEGIN RSA PRIVATE KEY-----
MIICXgIBAAKBgQDPmt5kitrIMyCp14MxGVSymoWnobd1M7aprIQks97bfYUtlmXl
P3KVJJ8oaMpP20QcPmASit0mpev/C17UiDhJKm5bvxI3R70Fa7zb8+7kEY5BaHxh
E9dCyIC+No/cCItPrKTidgzJY2xJWJPtzKrcJTsKYD+LVDrDTTHnlKRE/QIDAQAB
AoGBAMhiuereRFMdVpfZl54azzAeCohFhikwo02fYWVz4NaHj7bzBqOC3Hys/pso
mq79+/4vDtiXBOO7H9HsDbqjSX9HrIpEBH6GtzlCP60etw6lUfRAYx79veqfQZ+o
kB1vClqVExENgK9fZJiKLHhoZbAri6JTh8rxZASr8nvbg8iBAkEA975eI8MyP7+L
jjru/PzL5ffxC7xCgq7Vej7K99VpP2Qloh+3dXUFkkLruPHzCgXjlNFVeDWgNqCb
fJKEbN3cTQJBANaGDoVCCwQIhCFg8A4NiP0eNhBlTx5TtAhygFw/WWYX5pjy6Wx6
Bkysdj3tjKHOrRu9tH0ovOMOOI2Z2AW1o3ECQG3rwy0u8F6kiDEFKQCK0ZUpm4PP
ddsx43on3jp0MAx2TNRQKkAtOdmZY6ldgK5TypQ/BSMe+AUE4bg18hezoIkCQQCr
kIj6YAgpUJpDi6BQzNha/pdkY3F0IqMgAlrP68YWlVTRy6uNGsYA+giSnHHVUlI1
lnFLi5IM0Om/rWMLpemxAkEA3MwnyOTcYkjVm6/1q2D2If1T4rddCckaoQSp/GEs
XQRYOlo58UohVmf9zCCjj3gYWnk9Lo5+hgrmqDPBBBdKnw==
-----END RSA PRIVATE KEY-----