libgloss: fix isatty() and ttyname() — use TCGETS instead of TIOCGPGRP with NULL

libgloss: fix isatty() and ttyname() — use TCGETS instead of TIOCGPGRP with NULL

isatty() in syscalls.c used ioctl(TIOCGPGRP, 0) which always failed
because the kernel rejects NULL user_arg with -EFAULT. This caused
bash to think stdin was not a terminal, running in non-interactive
mode (no prompt, no line editing).

Fix: use TCGETS (0x5401) with a stack-allocated termios struct instead.
TCGETS succeeds on any tty regardless of process group state — this
is the standard POSIX way to test for a terminal.

Also fix ttyname() in posix_stubs.c which had the same bug.