sys: fix, work on DOS versions that clobber di in int 25h/26h

Running on DR-DOS v7.03, the int 25h call zeroes di leading to a crash
here when di is expected to preserve the stack pointer. So push di twice
and pop it twice. The second pop always gets the saved sp, regardless of
whether int 21h or int 25h/26h are used.

Refer to SvarDOS/edrdos#57 (comment)

sys/sys.c

@@ -122,7 +122,12 @@ int int86(int ivec, union REGS *in, union REGS *out)
 {
   /* must save sp for int25/26 */
   asm("mov %5, (1f+1); jmp 0f; 0:push %%ds; mov %%di, %%dx; mov %%sp, %%di;"
-      "1:int $0x00; mov %%di, %%sp; pop %%ds; sbb %0, %0" :
+      "push %%di; push %%di;"
+      /* push twice to work both for int 25h/26h and int 21h */
+      "1:int $0x00; pop %%di; pop %%di;"
+      /* second pop always reads the correct SP value.
+         the first pop may read the FL left on stack. */
+      "mov %%di, %%sp; pop %%ds; sbb %0, %0" :
       "=r"(out->x.cflag),
       "=a"(out->x.ax), "=b"(out->x.bx), "=c"(out->x.cx), "=d"(out->x.dx) :
       "q"((unsigned char)ivec), "a"(in->x.ax), "b"(in->x.bx),