fixed the "dot" (i.e. '.') command, to properly re-init its

fixed the "dot" (i.e.  '.') command, to properly re-init its
buffer between commands, not recalculate its length all the time,
and to not reallocate its fixed-length buffer frequently.

fixed the 'c', 'd', and 'y' commands to properly handle both motions
that are just part of a line, and those that are multiline.  for
instance, now "c%" and "2cw" work correctly, whether the motions span
lines or not.

added some permissble motions for 'c', 'd', and 'y':  namely 'h',
'l', SPACE, BS, and DEL