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

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

src/common/buf.c

@@ -0,0 +1,23 @@
+#include "buf.h"
+
+void buf_init(struct buf *s)
+{
+	s->alloc = 256;
+	s->buf = malloc(s->alloc);
+	s->buf[0] = '\0';
+	s->len = 0;
+}
+
+void buf_add(struct buf *s, const char *data)
+{
+	if (!data || data[0] == '\0')
+		return;
+	int len = strlen(data);
+	if (s->alloc <= s->len + len + 1) {
+		s->alloc = s->alloc + len;
+		s->buf = realloc(s->buf, s->alloc);
+	}
+	memcpy(s->buf + s->len, data, len);
+	s->len += len;
+	s->buf[s->len] = 0;
+}

src/common/meson.build

@@ -0,0 +1,3 @@
+labwc_sources += files(
+  'buf.c',
+)