login: Move libutil into libc

The symbols forkpty, login, login_tty, logout, logwtmp, openpty
were moved using scripts/move-symbol-to-libc.py.

This is a single commit because most of the symbols are tied together
via forkpty, for example.

Several changes to use hidden prototypes are needed.  This commit
also updates pseudoterminal terminology on modified lines.

For 390 (31-bit), this commit follows the existing style for the
compat symbol version creation.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>