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/flag.h

@@ -78,6 +78,7 @@ struct flag {
 #define PARANOID_BREAKWAND  0x0080
 #define PARANOID_WERECHANGE 0x0100
 #define PARANOID_EATING     0x0200
+#define PARANOID_SWIM       0x0400
     int pickup_burden; /* maximum burden before prompt */
     int pile_limit;    /* controls feedback when walking over objects */
     char discosort;    /* order of dodiscovery/doclassdisco output: o,s,c,a */
@@ -466,6 +467,8 @@ enum runmode_types {
 /* continue eating: prompt given _after_first_bite_ when eating something
    while satiated */
 #define ParanoidEating ((flags.paranoia_bits & PARANOID_EATING) != 0)
+/* Prevent going into lava or water without explicitly forcing it */
+#define ParanoidSwim ((flags.paranoia_bits & PARANOID_SWIM) != 0)
 
 /* command parsing, mainly dealing with number_pad handling;
    not saved and restored */