Ensure backwards compatibility with libpaper 1.1 by returning a value that
must be free()'d.

Taken from upstream:

  https://github.com/rrthomas/libpaper/commit/9a4f7cdd6b749fd9d08ec92b6e3b434f7d322b6d

diff --git a/lib/libpaper.c.in.in b/lib/libpaper.c.in.in
index aa86d06..19e3332 100644
--- a/lib/libpaper.c.in.in
+++ b/lib/libpaper.c.in.in
@@ -316,9 +316,9 @@ const char *defaultpapername(void) {
     return paperstr;
 }
 
-/* Alias for defaultpapername. */
+/* Alias for defaultpapername; its return value must be freed! */
 const char *systempapername(void) {
-    return defaultpapername();
+    return strdup(defaultpapername());
 }
 
 /* Get the default paper size. */
diff --git a/lib/paper.h b/lib/paper.h
index c940bed..fe5d4be 100644
--- a/lib/paper.h
+++ b/lib/paper.h
@@ -123,8 +123,8 @@ const struct paper *defaultpaper(void);
 const char *defaultpapername(void);
 
 /*
- * Deprecated, only for backwards compatibility; an alias for
- * defaultpapername().
+ * Deprecated, only for backwards compatibility; does the same as
+ * defaultpapername(), but returns a value that must be freed.
  */
 const char *systempapername(void);