bug#34821: discard_input_tty does not discard pending input, resulting in garbage inserted into the buffer

[Platon Pronko]

Found two problems with current workarounds.

1. Simply reading and discarding events before xterm--query call still leaves 
small timing window while single typed character breaks response parsing and 
garbage still ends up in the buffer.

2. Thus async querying is prefererrable, and it works most of the time, but sometimes 
"65;5403;1c" ends up in input buffer (even if no characters are typed at all).