Create a text entry for default cylinder in preferences

We don't do anything with this just yet.
And frankly, a drop down of the cylinders that we know of would be even
better. But hey, it's a start.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

pref.h

@@ -18,6 +18,7 @@ typedef struct {
 struct preferences {
 	const char *divelist_font;
 	const char *default_filename;
+	const char *default_cylinder;
 	double font_size;
 	partial_pressure_graphs_t pp_graphs;
 	short mod;