src/config/rcxml.c: parse xml from buffer

Avoid unit tests writing to/from files by using xmlParseMemory() instead
of xmlReadFile().

include/buf.h

@@ -0,0 +1,23 @@
+/*
+ * Very simple C buffer implementation
+ *
+ * Copyright Johan Malm 2020
+ */
+
+#ifndef BUF_H
+#define BUF_H
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct buf {
+	char *buf;
+	int alloc;
+	int len;
+};
+
+void buf_init(struct buf *s);
+void buf_add(struct buf *s, const char *data);
+
+#endif /* BUF_H */

include/rcxml.h

@@ -4,6 +4,8 @@
 #include <stdio.h>
 #include <stdbool.h>
 
+#include "buf.h"
+
 struct rcxml {
 	bool client_side_decorations;
 };
@@ -11,6 +13,7 @@ struct rcxml {
 extern struct rcxml rc;
 
 void rcxml_init(struct rcxml *rc);
+void rcxml_parse_xml(struct buf *b);
 void rcxml_read(const char *filename);
 void rcxml_set_verbose(void);