Fix warnings.

Fix warnings.

* gmarkup.c: Fix warnings.

* guniprop.c, gunidecomp.c: Make warnings go away by using
GPOINTER_TO_INT() instead of (int).

* gcompletion.[ch]: Add g_completion_set_compare(),
to allow (for example) using case-insensitive completion.

* gobject/gsignal.c: Fix warnings about possible use of uninitialized
variables, and fix logic that would leave 'node' unset in cases
that it might be used in.

* gobject/glib-genmarshal.c: Fix warning about printf format.