simplify pty allocation and implement pty logging

simplify pty allocation and implement pty logging

lxc-attach allocated a pty in a manner that relied on ts->stdinfd and
ts->stdoutfd to be set. We had to resort to a trick to get it working when
output is redirected. The new implementation gets rid of the black magic and
also simplifies the code.

This commit also implements pty logging for lxc-attach. Users will now be able
to log commands and corresponding output to a file given that lxc-attach
allocates a pty.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>