Fix job control TIOCSPGRP: allow session leader to claim TTY/PTY

Fix job control TIOCSPGRP: allow session leader to claim TTY/PTY

When fulltest runs from the shell, the TTY's tty_session_id is already
set to the shell's session. The test's leader child calls setsid() then
TIOCSPGRP, which fails with EPERM because the new session doesn't match
tty_session_id. The kernel never allowed a new session leader to claim
the TTY as its controlling terminal.

Fix: in TIOCSPGRP handler for both tty.c and pty.c, allow a session
leader (session_id == pid) to claim the terminal by updating the
session when it explicitly sets its own pgrp as foreground. This
matches POSIX semantics where a session leader may establish a new
controlling terminal.

Test results: 103/103 smoke test PASS, 16/16 battery PASS.