Store drop_stone_mode, bottomsac, decosac in prefs

The values for drop_stone_mode, bottomsac, and decosac are typically the
kind of personal data specific to a diver that is unlikely to change
from one dive plan to the next.

This patch stores/restores them to/from the preferences file. For this,
it adds bottomsac and decosac to the prefs structure; drop_stone_mode
was already there, though not stored/restored.

Signed-off-by: Gaetan Bisson <bisson@archlinux.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

subsurfacestartup.c

@@ -42,6 +42,8 @@ struct preferences default_prefs = {
 	.decopo2 = 1600,
 	.doo2breaks = false,
 	.drop_stone_mode = false,
+	.bottomsac = 20,
+	.decosac = 17,
 	.show_pictures_in_profile = true
 };