plugins CHANGE unify naming in plugins API

All the symbols connected with type plugins are now prefixed by
lyplg_type_ and symbols connected with extensions have lyplg_ext_
prefix.
diff --git a/src/log.c b/src/log.c
index d4fbf58..4cfb01c 100644
--- a/src/log.c
+++ b/src/log.c
@@ -584,7 +584,7 @@
 }
 
 API void
-lyext_log(const struct lysc_ext_instance *ext, LY_LOG_LEVEL level, LY_ERR err_no, const char *path, const char *format, ...)
+lyplg_ext_log(const struct lysc_ext_instance *ext, LY_LOG_LEVEL level, LY_ERR err_no, const char *path, const char *format, ...)
 {
     va_list ap;
     char *plugin_msg;