[misc] Add misc_xml.c to wrap mxml, fixes XML reading of CDATA (et al)

The change removes all direct calls to mxml from the modules that need an XML
parser (lastfm.c, pipe.c, rssscanner.c and httpd_rsp.c).

Even with the help of mxml, reading XML is hard, so a layer is added which
helps deal with stuff like whitespace and CDATA. This should make OwnTone more
resilient to any XML variations it might receive.

The changes fixes issue #1677.

src/misc_xml.h

@@ -0,0 +1,56 @@
+#ifndef SRC_MISC_XML_H_
+#define SRC_MISC_XML_H_
+
+// This wraps mxml and adds some convenience functions. This also means that
+// callers don't need to concern themselves with changes and bugs in various
+// versions of mxml.
+
+typedef void xml_node;
+
+// Wraps mxmlSaveAllocString. Returns NULL on error.
+char *
+xml_to_string(xml_node *top);
+
+// Wraps mxmlNewXML and mxmlLoadString, so creates an xml struct with the parsed
+// content of string. Returns NULL on error.
+xml_node *
+xml_from_string(const char *string);
+
+// Wraps mxmlNewXML and mxmlLoadFile, so creates an xml struct with the parsed
+// content of string. Returns NULL on error.
+xml_node *
+xml_from_file(const char *path);
+
+// Wraps mxmlDelete, which will free node + underlying nodes
+void
+xml_free(xml_node *top);
+
+// Wraps mxmlFindPath.
+xml_node *
+xml_get_node(xml_node *top, const char *path);
+
+// Wraps mxmlGetNextSibling, but only returns sibling nodes that have the same
+// name as input node.
+xml_node *
+xml_get_next(xml_node *top, xml_node *node);
+
+// Wraps mxmlFindPath and mxmlGetOpaque + mxmlGetCDATA. Returns NULL if nothing
+// can be found.
+const char *
+xml_get_val(xml_node *top, const char *path);
+
+// Wraps mxmlFindPath and mxmlElementGetAttr. Returns NULL if nothing can be
+// found.
+const char *
+xml_get_attr(xml_node *top, const char *path, const char *name);
+
+xml_node *
+xml_new_node(xml_node *parent, const char *name, const char *val);
+
+xml_node *
+xml_new_node_textf(xml_node *parent, const char *name, const char *format, ...);
+
+void
+xml_new_text(xml_node *parent, const char *val);
+
+#endif /* SRC_MISC_XML_H_ */