some spelling corrections

The pull request included some changes that were neither accidental nor
unintentional, so only a subset of the changes from pull request #869
submitted by klorpa were manually applied.

behaviour  -> behavior
speach     -> speech
knowlege   -> knowledge
incrments  -> increments
stethscope -> stethoscope
staiway    -> stairway
arifact    -> artifact
extracing  -> extracting

The uses of "iff" were left alone.

Close #869

src/hack.c

@@ -1404,7 +1404,7 @@ is_valid_travelpt(coordxy x, coordxy y)
    (all failures and most successful escapes leave hero at original spot) */
 static boolean
 trapmove(
-    coordxy x, coordxy y, /* targetted destination, <u.ux+u.dx,u.uy+u.dy> */
+    coordxy x, coordxy y, /* targeted destination, <u.ux+u.dx,u.uy+u.dy> */
     struct trap *desttrap) /* nonnull if another trap at <x,y> */
 {
     boolean anchored = FALSE;
@@ -1947,7 +1947,7 @@ domove_fight_empty(coordxy x, coordxy y)
         if (boulder) {
             Strcpy(buf, ansimpleoname(boulder));
         } else if (Underwater && !is_pool(x, y)) {
-            /* Underwater, targetting non-water; the map just shows blank
+            /* Underwater, targeting non-water; the map just shows blank
                because you don't see remembered terrain while underwater;
                although the hero can attack an adjacent monster this way,
                assume he can't reach out far enough to distinguish terrain */