Fix clist limits. They were usually one too low. E.g., for a limit of
TTYHOG = 1024 bytes, 10 cblocks were reserved. This was thought to
provide 10 * CBSIZE = 1080 bytes of buffering, but if the head pointer
is at the end of a cblock, then it only provides 1 + 9 * CBSIZE = 973
bytes of buffering. This caused serious data loss for ptys because the
flow control is deterministic and requires at least TTYHOG bytes of
buffering. For ttys, if input flow control is used then there is
usually enough slop in the high watermark to avoid problems, and if
input flow control isn't used then a limit of 973 is not much different
from a limit of 1024.