terminal: use /dev/ptmx when allocating pty devices from devpts instances we didn...

terminal: use /dev/ptmx when allocating pty devices from devpts instances we didn't mount ourselves

When we aren't told what devpts instance to allocate from we assume it
is the one in the caller's mount namespace.
This poses a slight complication, a lot of distros will change
permissions on /dev/ptmx so it can be opened by unprivileged users but
will not change permissions on /dev/pts/ptmx itself. In addition,
/dev/ptmx can either be a symlink, a bind-mount, or a separate device
node. So we need to allow for fairly lax lookup.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>