fix github issue #354 - stealarm() impossible

A thieving monster could be killed while the hero was busy taking
off armor which needs multiple turns (normally a suit) and if that
happened on the same turn as the take-off finished, the warning
"stealarm(): dead monster stealing" was issued.  Cited case was
having the thief be killed by a stinking cloud but it could happen
if the death was caused by a pet or by some other monster trying
to attack the hero.  If the thief died sooner, the situation was
silently ignored.  So this could have been fixed by just getting
rid of the impossible() feedback.

'stealmid' and 'stealoid' should have been static in steal.c rather
than global and as such should have been moved into 'struct g'.
This moves them there and then takes advantage of having access to
'stealmid' outside of steal.c.  That's just a minor optimization
since m_detach() could call new thiefdead() unconditionally and the
latter could check whether the dead monster matches 'stealmid'.

Fixes #354

doc/fixes37.0

@@ -1,4 +1,4 @@
-$NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.227 $ $NHDT-Date: 1590879610 2020/05/30 23:00:10 $
+$NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.228 $ $NHDT-Date: 1591017415 2020/06/01 13:16:55 $
 
 General Fixes and Modified Features
 -----------------------------------
@@ -190,6 +190,8 @@ when hold_another_object fails while hero is swallowed, drop the item into
 change mkclass() to usually honor (always honor for L class) the hell-only and
 	never-in-hell monster creation flags; no more achi-lich in the Castle
 	(nor master lich there unless demilich gets a potion of gain level)
+thieving monster could be killed while hero was removing armor, triggering
+	warning "stealarm(): dead monster stealing" when taking-off finished
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository