common: add generic input-line interface, and an implementation of it for text files

common/line-source.c

@@ -1,6 +1,6 @@
 #include <ivy/line-source.h>
 
-enum ivy_line_source_status ivy_line_source_readline(
+enum ivy_status ivy_line_source_readline(
 	struct ivy_line_source *src, char *buf, size_t count, size_t *nr_read,
 	const char *prompt)
 {
@@ -8,5 +8,5 @@ enum ivy_line_source_status ivy_line_source_readline(
 		return src->s_readline(src, buf, count, nr_read, prompt);
 	}
 
-	return IVY_LS_ERR_NOSYS;
+	return IVY_ERR_NOT_SUPPORTED;
 }