Before this change, we would canonicalize any partial input if the new
local mode is not ICANON, but that's about it. If we were switching
from -ICANON -> ICANON, or if VEOF/VEOL changes, then our internal canon
accounting would be wrong.

The main consequence of this is that in ICANON mode, we would potentially
hang a read(2) longer if the new VEOF/VEOL appears later in the buffer,
and FIONREAD would be similarly wrong as a result.