Fix quit on ctrlc, race panic, atomic load align in session IO

[xacrimon]

lib/srv/termmanager.go is responsible for dealing with most session IO and handles behaviours like multiplexing. This PR fixes the following bugs:

• Unaligned integers being used as atomics Teleport panics with unaligned 64-bit atomic operation on 32bit ARM #11106
• CTRL-C causing the reader goroutine to quit early CTRL-C causing sessions to freeze on 9.0 #11092
• A race condition that could case a panic since we didn't synchronise and check the correct flags prior to sending over a potentially closed channel.

This PR does remove some usage of atomics in termmanager and instead locks additional fields under the mutex. I didn't observe any noticable performance drop here and this would only become a problem if you had hundreds of users typing in a session at the same time. This makes the code somewhat easier to reason about but we'll have to look into that again if it becomes a problem at some point in the future.

[rosstimothy]

It doesn't look like lib/srv/termmanager.go has much test coverage at all. Could we at least add some for the issues this is fixing to prevent any regressions here?

[xacrimon]

@rosstimothy I wasn't able to trigger the race condition that was potentially possible due to tight tolerances or some other reason like optimization but I've added tests for CTRLC capture and atomic alignment. Please have another look