type CHANGE join ly_err_new() and ly_err_msg_create()

Keep only ly_err_new(), but simplify and extend its functionality
according to ly_err_msg_create() to be able to handle format string
with possible additional parametrs to modify the error message.

The error message is now also always duplicated to avoid any special
cases such as memory allocation failure message. Its usability was
questionable since it was necessary to allocate error record anyway.
diff --git a/src/log.c b/src/log.c
index af28cdf..0927430 100644
--- a/src/log.c
+++ b/src/log.c
@@ -118,25 +118,52 @@
     return NULL;
 }
 
-API struct ly_err_item *
-ly_err_new(LY_LOG_LEVEL level, LY_ERR no, LY_VECODE vecode, char *msg, char *path, char *apptag)
+API LY_ERR
+ly_err_new(struct ly_err_item **err, LY_ERR ecode, LY_VECODE vecode, char *path, char *apptag, const char *err_msg, ...)
 {
-    struct ly_err_item *eitem;
+    char *msg = NULL;
+    struct ly_err_item *e;
 
-    eitem = malloc(sizeof *eitem);
-    LY_CHECK_ERR_RET(!eitem, LOGMEM(NULL), NULL);
-    eitem->prev = eitem;
-    eitem->next = NULL;
+    if (!err || ecode == LY_SUCCESS) {
+        /* nothing to do */
+        return ecode;
+    }
+
+    e = malloc(sizeof *e);
+    LY_CHECK_ERR_RET(!e, LOGMEM(NULL), LY_EMEM);
+    e->prev = (*err) ? (*err)->prev : e;
+    e->next = NULL;
+    if (*err) {
+        (*err)->prev->next = e;
+    }
 
     /* fill in the information */
-    eitem->level = level;
-    eitem->no = no;
-    eitem->vecode = vecode;
-    eitem->msg = msg;
-    eitem->path = path;
-    eitem->apptag = apptag;
+    e->level = LY_LLERR;
+    e->no = ecode;
+    e->vecode = vecode;
+    e->path = path;
+    e->apptag = apptag;
 
-    return eitem;
+    if (err_msg) {
+        va_list print_args;
+
+        va_start(print_args, err_msg);
+
+        if (vasprintf(&msg, err_msg, print_args) == -1) {
+            /* we don't have anything more to do, just set err_msg to NULL to avoid undefined content,
+             * still keep the information about the original error instead of LY_EMEM or other printf's error */
+            msg = NULL;
+        }
+
+        va_end(print_args);
+    }
+    e->msg = msg;
+
+    if (!(*err)) {
+        *err = e;
+    }
+
+    return e->no;
 }
 
 API struct ly_err_item *
@@ -166,9 +193,7 @@
     /* clean the error list */
     for (i = (struct ly_err_item *)ptr; i; i = next) {
         next = i->next;
-        if (i->msg && strcmp(i->msg, LY_EMEM_MSG)) {
-            free(i->msg);
-        }
+        free(i->msg);
         free(i->path);
         free(i->apptag);
         free(i);