Tune posix_spawn() successful fix

Author: jart

libc/proc/posix_spawn.c

@@ -350,7 +350,7 @@ static textwindows errno_t posix_spawn_nt_impl(
   // figure out flags
   uint32_t dwCreationFlags = 0;
   short flags = attrp && *attrp ? (*attrp)->flags : 0;
-  if (flags & POSIX_SPAWN_SETPGROUP)
+  if (flags & (POSIX_SPAWN_SETPGROUP | POSIX_SPAWN_SETSID))
     dwCreationFlags |= kNtCreateNewProcessGroup;
 
   // create process startinfo