DosFlatToSel() needs to save off registers.

It's not syscall calling convention, and CSet/2 generates thunks that call
this function but expect %ecx to not be altered by it. Save off a bunch,
just in case.

native/doscalls.c

@@ -657,13 +657,26 @@ ULONG _DosFlatToSel(PVOID ptr)
 } // _DosFlatToSel
 
 // DosFlatToSel() passes its argument in %eax, so a little asm to bridge that...
+// Note that this isn't syscall calling conventions at all; CSet/2 generates
+//  thunking code that expects this function to preserve %ecx, so we'll just
+//  go ahead and save a bunch of stuff, just in case.
 __asm__ (
     ".globl DosFlatToSel  \n\t"
     ".type	DosFlatToSel, @function \n\t"
     "DosFlatToSel:  \n\t"
-    "    pushl %eax  \n\t"
+    "    pushl %ebx  \n\t"  // save off a bunch of stuff. Better safe than sorry.
+    "    pushl %ecx  \n\t"
+    "    pushl %edx  \n\t"
+    "    pushl %edi  \n\t"
+    "    pushl %esi  \n\t"
+    "    pushl %eax  \n\t"  // the actual argument to DosFlatToSel.
     "    call _DosFlatToSel  \n\t"
-    "    addl $4, %esp  \n\t"
+    "    addl $4, %esp  \n\t"  // clear argument to DosFlatToSel.
+    "    popl %esi  \n\t"
+    "    popl %edi  \n\t"
+    "    popl %edx  \n\t"  // save off a bunch of stuff.
+    "    popl %ecx  \n\t"
+    "    popl %ebx  \n\t"
     "    ret  \n\t"
 	".size	_DosFlatToSel, .-_DosFlatToSel  \n\t"
 );