[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/Makefile.am

@@ -107,6 +107,7 @@ owntone_SOURCES = main.c \
 	artwork.c artwork.h \
 	misc.c misc.h \
 	misc_json.c misc_json.h \
+	misc_xml.c misc_xml.h \
 	rng.c rng.h \
 	smartpl_query.c smartpl_query.h \
 	player.c player.h \
@@ -128,7 +129,6 @@ owntone_SOURCES = main.c \
 	$(MPD_SRC) \
 	listener.c listener.h \
 	commands.c commands.h \
-	mxml-compat.h \
 	outputs/plist_wrap.h \
 	$(LIBWEBSOCKETS_SRC) \
 	$(GPERF_SRC) \