Prevent evaluation of backticks while discarding unwanted arguments

Prevent evaluation of backticks while discarding unwanted arguments
after an unknown or failed psql backslash command, and also while
discarding "extra" arguments of a putatively valid backslash command.
In the case of an unknown/failed command, make sure we discard the
whole rest of the line, rather than trying to resume at the next
backslash.  Per discussion with Thomer Gil.