When clearing the [CR]LF in a line continuation, we would continue
replacing any [CR|LF|HT|SP] characters up until the end of the buffer,
possibly overwriting later [CR]LFs. Fix this by only unconditionally
overwrite one [CR]LF, and then only replace [HT|SP] with SP to keep with
previous behaviour.

Update r00494.vtc to include multiple line continuations to make sure they
are parsed.