[Toybox] [New toy] rudimentary getty

[ibid.ag at gmail.com]

Here's a basic getty.
It just reopens the current tty in such a way as to allow blocking code
to work, then calls the login program of your choice (-l).
/etc/issue is ignored, so -i is a nop.
It does NOT modify the environment (including setting TERM), support
baud rate setting or autodetection, do anything with modems, prompt for
any user input, or open new ttys.
Supporting 
getty ttyN $TERM
should be something like this...
char * prepend(char *arg, char *pfix)
{
  if (!arg) return NULL;
  int alen = strlen(arg), plen = strlen(pfix);
  char * ret = xzalloc(alen + plen + 1);
  strcpy(ret, pfix);
  strcpy(ret+plen, arg);
  return ret;
}
  

  char *tty = prepend(toys.optargs[0], "/dev/")
  if (!tty) tty = ttyname(0);
  if (toys.optc > 1) {
    char *termname = prepend(toys.optargs[1], "TERM=");
    if (termname) putenv(termname);
  }

--
With this, I can boot to a login prompt using only a shell and a toybox
binary (with several pending applets enabled, and a hacked version of
one of the many submissions for mount).

I also have a small applet (resolve_modalias) that is mainly useful for 
testing, which I've sent a couple times to illustrate issues;
maybe sometime I should see about putting the core code into a library
function so modprobe and modinfo can share it....

Thanks,
Isaac Dunham
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