temporary? fix for github issue #730 - stairs \

placed on lava spot on Valkyrie goal level

Reported by k2, arriving at the final level of the Valkyrie quest
can issue a recently added impossible
| mkstairs: placing stairs up on molten lava at <68,13>
The report said it was easy to reproduce, but it took me multiple
tries (so not hard to do, but not a sure thing on any given attempt).

The stairs on that level are placed at specific coordinates that
are outside the pre-mapped area, so there's no guarantee that their
spot will be suitable for stairs.  The underlying terrian changes
from lava to stair, but only after the warning about molten lava.

This hack solves that particular level but is not a general solution
for this type of thing.  When about to make stairs on a lava spot,
change that spot to normal floor first.  Plus 50:50 chance to change
each adjacent lava spot to floor too so that there's decent chance
for some elbow room upon arrival.

Also, turn the no-flip attribute off for that level so that 'fixed'
location of the stairs can occur in four different places.

Fixes #730

doc/fixes3-7-0.txt

@@ -1147,6 +1147,8 @@ sequencing confusion: picking an item when viewing inventory and picking an
 	action to do with it caused the inventory command to use time, then
 	on next turn the action was performed without taking any time
 program would access freed memory if charging caused a ring to explode
+arriving on Valkyrie quest final level could produce impossible warning
+	"mkstairs: placing stairs up on molten lava at <68,13>"
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support