The MYSQL_* authentication return codes are now in gw_authenticator.h so
that all authenticators can use them. Also dropped the MYSQL_ prefix from
the return codes and added AUTH_INCOMPLETE for a generic
authentication-in-progress return code.
The authenticators can now declare the authentication plugin name. Right
now this is only relevant for MySQL authentication but for example the
HTTP module could implement both Basic and Digest authentication.
Each protocol module now resides in a subdirectory of its own.
The name of the subdirectory is the same as the name of the
library in the module's CMakeLists.txt file.
