8086: implement stack overflow error condition

Also, remove RSP underflow check from next for the same reason as
in the previous commit in z80.

blk/283

@@ -2,7 +2,8 @@ H@ ORG ! ( STABLE ABI )
 0 JP, ( 00, main ) NOP, ( unused ) NOP, NOP, ( 04, BOOT )
 NOP, NOP, ( 06, uflw ) NOP, NOP, ( 08, LATEST ) NOP, ( unused )
 0 A, 0 JP, ( 0b, EXIT ) NOP, ( unused )
-0 JP, ( RST 10 )  NOP, NOP, NOP, NOP, NOP, NOP, NOP, ( unused )
+0 JP, ( RST 10 )  NOP, NOP, ( 13, oflw )
+NOP, NOP, NOP, NOP, NOP, ( unused )
 0 JP, ( 1a, next ) NOP, NOP, NOP, ( unused )
 0 JP, ( RST 20 ) 0 A, 0 JP, ( 23, (n) ) NOP, ( unused )
 0 JP, ( RST 28 ) 0 A, 0 JP, ( 2b, (s) ) NOP, ( unused )

blk/805

@@ -2,10 +2,10 @@ VARIABLE lblexec VARIABLE lblnext
 H@ ORG !
 JMPn, 0 A,, ( 00, main ) 0 A, ( 03, boot driveno )
 0 A,, ( 04, BOOT )
-0 A,, ( 06, uflw ) 0 A,, ( 08, LATEST ) 0 A, ( unused )
-0 A, JMPn, 0 A,, ( unused )
-0 A,, ( unused ) JMPn, 0 A,,  ( unused )
-JMPn, 0 A,, ( unused ) 0 A, 0 A,, ( unused )
+0 A,, ( 06, uflw ) 0 A,, ( 08, LATEST ) 0 A,, ( unused )
+JMPn, 0 A,, ( 0b, EXIT )
+0 A,, 0 A,, ( unused ) 0 A,, ( 13, oflw )
+0 A,, 0 A,, 0 A, ( unused )
 JMPn, 0 A,, ( 1a, next ) 0 A,, 0 A,, 0 A,, ( unused )
 0 A, JMPn, 0 A,, ( 23, (n) )
 0 A,, 0 A,, ( unused )

blk/808

@@ -1,8 +1,9 @@
 lblnext BSET PC 0x1d - ORG @ 0x1b + ! ( next )
-    ( RSP check )
-    AX RS_ADDR MOVxI, BP AX CMPxx,
-    IFC, ( BP < RS_ADDR )
-        DI 0x06 MOVxm, JMPs, L1 FWRs ( execute )
+    ( ovfl check )
+    BP SP CMPxx,
+    IFNC, ( BP >= SP )
+        SP PS_ADDR MOVxI, BP RS_ADDR MOVxI,
+        DI 0x13 ( oflw ) MOVxm, JMPs, L1 FWRs ( execute )
     THEN,
     DI DX MOVxx, ( <-- IP ) DX INCx, DX INCx,
     DI [DI] MOVx[], ( wordref )