A pipeline of two sudo invocations causes TTY contention

[squell]

From #1649, reported by @MoyashiWithDevice:

When multiple sudo processes are piped together (e.g. sudo a | sudo b),
both processes open /dev/tty and call poll()+read() concurrently. Because
the terminal delivers each byte to only one reader, keystrokes from the
user get split between the two processes, resulting in both receiving an
incomplete password and authentication failure.

[squell]

Note that if we serialize the TTY access as in #1649, a user would still need to authenticate twice; i.e. if sudo b wins the race for the lock, sudo a can't benefit from the presence of the timestamp file that is created after the authentication succeeds in sudo b.

We can probably avoid that by moving the lock on the TTY higher up in the call-chain, e.g. before checking the timestamp file. I can also imagine this has unforseen negative consequences.