libyang FEATURE introduce and use new ly_bool type

To indicate simple flags or true/false return values, use a standalone
ly_bool type.

We do not use stdbool's bool type to avoid need to mimic all its
features on platforms that do not provide it. ly_bool is just a simple
rename for uint8_t and the reason to use it is only a better readability
of the meaning of the variables or function's return values.
diff --git a/src/parser_stmt.c b/src/parser_stmt.c
index af0feee..3c36fbd 100644
--- a/src/parser_stmt.c
+++ b/src/parser_stmt.c
@@ -32,7 +32,8 @@
 static LY_ERR
 lysp_stmt_validate_value(struct lys_parser_ctx *ctx, enum yang_arg val_type, const char *val)
 {
-    uint8_t prefix = 0, first = 1;
+    uint8_t prefix = 0;
+    ly_bool first = 1;
     uint32_t c;
     size_t utf8_char_len;