Add paranoid:swim to prevent typoing into water or lava

In the name of accessibility: Prevent moving into dangerous liquids.

Now with themed rooms, water and lava are more common, and it's
unreasonable to expect blind players to check every step for those.
With paranoid:swim, just prevent normal walking into those liquids,
unless you prefix the movement with 'm', or if the liquid would not
harm you.

Doesn't completely prevent an accidental dunking - for example
if the hero is impaired or couldn't see the liquid.

This comes from xNetHack by copperwater <aosdict@gmail.com>
with some changes to the code.

include/context.h

@@ -150,6 +150,7 @@ struct context_info {
     boolean botlx;       /* print an entirely new bottom line */
     boolean door_opened; /* set to true if door was opened during test_move */
     boolean enhance_tip; /* player is informed about #enhance */
+    boolean swim_tip;    /* player was informed about walking into water */
     struct dig_info digging;
     struct victual_info victual;
     struct engrave_info engraving;