dos/com&sys: correctly set carry bit by using "sub si,-xx" instead of "add si,xx"

src/stub/tmp/i086-dos16.com.bin.dump

@@ -32,7 +32,6 @@ Idx Name          Size      VMA       LMA       File off  Algn  Flags
  27 CALLTRI6      00000003  00000000  00000000  000000ee  2**0  CONTENTS, RELOC, READONLY
  28 CORETURN      00000001  00000000  00000000  000000f1  2**0  CONTENTS, READONLY
 SYMBOL TABLE:
-00000000 l    d  COMCUTPO	00000000 COMCUTPO
 00000000 l    d  NRV2B160	00000000 NRV2B160
 00000000 l    d  NRVDECO2	00000000 NRVDECO2
 00000000 l    d  CALLTR16	00000000 CALLTR16
@@ -44,6 +43,7 @@ SYMBOL TABLE:
 00000000 l    d  COMCALLT	00000000 COMCALLT
 00000000 l    d  COMMAIN2	00000000 COMMAIN2
 00000000 l    d  UPX1HEAD	00000000 UPX1HEAD
+00000000 l    d  COMCUTPO	00000000 COMCUTPO
 00000000 l    d  NRVDDONE	00000000 NRVDDONE
 00000000 l    d  NRVDRETU	00000000 NRVDRETU
 00000000 l    d  NRVDECO1	00000000 NRVDECO1
@@ -65,6 +65,7 @@ SYMBOL TABLE:
 00000000         *UND*	00000000 bytes_to_copy
 00000000         *UND*	00000000 copy_source
 00000000         *UND*	00000000 copy_destination
+00000000         *UND*	00000000 neg_e_len
 00000000         *UND*	00000000 calltrick_calls
 
 RELOCATION RECORDS FOR [COMMAIN1]:
@@ -73,7 +74,7 @@ OFFSET   TYPE              VALUE
 00000009 R_386_16          bytes_to_copy
 0000000c R_386_16          copy_source
 0000000f R_386_16          copy_destination
-0000001c R_386_8           COMCUTPO
+0000001c R_386_8           neg_e_len
 
 RELOCATION RECORDS FOR [COMMAIN2]:
 OFFSET   TYPE              VALUE