Fix serial putchar newline deadlock

[Courtney3141]

While making recent changes to the serial subsystem, I notice a potential deadlock in the _sddf_putchar function.

Prior to this PR, the function only signals the TX_CH if the queue is full after inserting a character, or if the character inserted was the FLUSH_CHAR. However, the function only actually inserts a character into the queue if it is not full, or in the case of a \n character, the queue has space for 2 characters.

This means that if the queue only has space for one character, and a \n is attempted to be inserted, the function will immediately return without signalling, and there is no guarantee the function signalled on its previous invocation. This is a potential deadlock, as the virtualiser has potentially not yet been signalled for the remaining characters in the queue, and the system is dependent on the client enqueuing another non \n character for a signal to occur.

This PR fixes this deadlock situation by signalling if either the queue is full, the flush character is seen, or if the queue is one from full, which eliminates the scenario that a signal is missed. It also stops any characters from being enqueued if the queue is 1 from full, to prevent against accidental double signals which would interfere with the multiplexing.

Merge after #305.