Fix async-process.c for OpenBSD

[rpx99]

O_CLOEXEC used in

async-process/src/async-process.c

Line 7 in 3b16b91

int fd = posix_openpt(O_RDWR | O_CLOEXEC | O_NOCTTY);

is not supported in OpenBSD, c.f.: https://man.openbsd.org/posix_openpt

This fixed it for me (recompile with bootstrap and copy like so cp static/amd64/OpenBSD/libasyncprocess.so /usr/local/lib):

static const char* open_pty(int *out_fd)
{
  int fd = posix_openpt(O_RDWR | O_NOCTTY);
  if (fd < 0) return NULL;
  if (fcntl(fd, F_SETFD, FD_CLOEXEC) == -1) {
    close(fd);
    return NULL;
  }
  
  if (grantpt(fd) == -1 || unlockpt(fd) == -1) {
    close(fd);
    return NULL;
  }
  //if (grantpt(fd) == -1 || unlockpt(fd) == -1) return NULL;
  //fcntl(fd, F_SETFD, FD_CLOEXEC);
  const char *name = ptsname(fd);
  if (name == NULL) {
    close(fd);
    return NULL;
  }
  *out_fd = fd;
  return name;
}

Is it possible to change the code please ?