grantpt: trust the kernel about pty group and permission mode

According to POSIX the grantpt() function does the following:

  The grantpt() function shall change the mode and ownership of the
  slave pseudo-terminal device associated with its master
  pseudo-terminal counterpart. The fildes argument is a file descriptor
  that refers to a master pseudo-terminal device. The user ID of the
  slave shall be set to the real UID of the calling process and the
  group ID shall be set to an unspecified group ID. The permission
  mode of the slave pseudo-terminal shall be set to readable and
  writable by the owner, and writable by the group.

Historically the GNU libc has been responsible to setup the permission
mode to 0620 and the group to 'tty' usually number 5, using the pt_chown
helper, badly known for its security issues. With the creation of the
devpts filesytem in the Linux kernel, this responsibility has been moved
to the Linux kernel. The system is responsible to mount the devpts
filesystem in /dev/pts with the options gid=5 and mode=0620. In that
case the GNU libc has nothing to do and pt_chown is not need anymore. So
far so good.

The problem is that by default the devpts filesystem is shared between
all mounts, and that contrary to other filesystem, the mount options are
honored at the second mount, including for the default mount options.
Given it corresponds to mode=0600 without gid parameter (that is the
filesystem GID of the creating process), it's common to see systems
where the devpts filesystem is mounted using these options. It is enough
to run a "mount -t devpts devpts /mychroot/dev/pts" to come into this
situation, and it's unfortunately wrongly used in a lot of scripts
dealing with chroots, or for creating virtual machines images.

When this happens the GNU libc tries to fix the group and permission
mode of the pty nodes, and given it fails to do so for non-root users,
grantpt() almost always fail. It means users are not able to open new
terminals.

This patch changes grantpt() to not enforce this anymore, while still
enforcing minimum security measures to the permission mode. Therefore
the responsibility to follow POSIX is now shared at the system level,
i.e. kernel + system scripts + GNU libc. It stops trying to change the
group, and makes the pty node readable and writable by the owner, and
writable by the group only when originally writable and when the group
is the tty one.

As a result, on a system wrongly mounted with gid=0 and mode=0600, the
pty nodes won't be accessible by the tty group, but the grantpt()
function will succeed and users will have a working system. The system
is not fully POSIX compliant (which might be an admin choice to default
to "mesg n" mode), but the GNU libc is not to blame here, as without the
pt_chown helper it can't do anything.

With this patch there should not be any reason left to build the GNU
libc with the --enable-pt_chown configure option on a GNU/Linux system.

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