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/rumors.c

@@ -893,7 +893,7 @@ init_CapMons(void)
      * CapMons[] init doesn't kick in until needed.  To force this name
      * dump, set DEBUGFILES to "CapMons" in your environment (or in
      * sysconf) prior to starting nethack, wish for a statue of an Archon
-     * and drop it if held, then step away and apply a stethscope towards
+     * and drop it if held, then step away and apply a stethoscope towards
      * it to trigger a message that passes "Archon" to the() which will
      * then call CapitalMon() which in turn will call init_CapMons().
      */