Shell doesn't always process input data when it arrives

[jimparis]

The shell doesn't always consume all input data when it arrives.
For example, when several lines worth of data are pasted simultaneously from the host, the shell will act as if it only received the first or second line. If the host sends even more input, then the shell starts to "catch up" and processes the third original pasted line as if it had just arrived.

Tested on nrf52 with shell over UART but this would affect any transport.

The exact behavior is messy depending on timing and whether the shell_uart RX buffer overflows -- basically the user visible effect is that the shell seems to drop input, then processes old input when new input comes in, etc.

This is fixed by PR #15224

[jakub-uC]

I've added @jimparis as a resolver because he already fixed it ;)