For the same reason we don't care about the state of
the buffer in clearundo() we can reset modflag in
clearbuf(), because after calling it we don't want
to know anything about the previous state.
When we discard the content of an undo buffer
is because we don't need it anymore, and it means
that we don't care about the modify state of the buffer
so we can reset the modflag in clearundo(), and
remove this assignation each time clearundo() is called.
getrhs() must remove \ excepts in the case of & and \d
(where d is a digit), because in this case are sequences
understood by addsub(), so addsub() must be able to see
them.
When a error occurs it is important to remove all the modifications
done by the offending command and restore the value of dot to the
state before of executing the command.
