fix issue #509 - shop 'glob' pricing segfault

A recent change made it possible for a glob to have its dknown flag
cleared and that exposed globby_bill_fixup() passing Null shopkeeper
to get_cost(), triggering a crash.

Make the routine that clears dknown/known/bknown/&c also be the
routine used to initialize those flags for a new object so that it
is the place that handles various special cases.  That hides the
shop bug again.

But also fix the shop bug even though it won't be triggered.

Fixes #509

doc/fixes37.0

@@ -1,4 +1,4 @@
-NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.530 $ $NHDT-Date: 1620590081 2021/05/09 19:54:41 $
+NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.532 $ $NHDT-Date: 1620861202 2021/05/12 23:13:22 $
 
 General Fixes and Modified Features
 -----------------------------------
@@ -665,6 +665,10 @@ if a <foo> corpse was set to revive as a <foo> zombie and corpse was partly
 	calculate zombie's hit points (depended upon how much had been eaten)
 undead turning magic revived corpses flagged as no-revive if they were being
 	carried but not if they were zapped while on the floor
+forgetting an item's known/dknown/bknown/&c settings when picked up by unseen
+	monster made it possible to trigger a latent shop bug with globs;
+	avoid clearing dknown flag for globs
+fix globby_bill_fixup to use shopkeeper instead of Null for glob pricing
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support