Bug 628937 - gracefully handle broken schemas

Implement the first of two features requested in the bug: when
encountering a broken .xml schema file, back out the changes in that
file and continue to parse other files.

This prevents a single broken .xml file from messing up GSettings for
everyone else.

Add a --strict option to get the old behaviour.  Use this from the test
cases.

gio/tests/gschema-compile.c

@@ -22,6 +22,7 @@ test_schema (gpointer data)
       gchar *path = g_build_filename (SRCDIR, "schema-tests", filename, NULL);
       gchar *argv[] = {
         "../glib-compile-schemas",
+        "--strict",
         "--dry-run",
         "--schema-file", path,
         (gchar *)test->opt,