Make Conque work with t_kb remapped to '\177'

When t_kb is set to '\177' (exe ":set t_kb=\177") Conque imap-s the key
twice and then tries to iunmap it twice too. Second attempt fails as
there is no mapping anymore.

The fix tries to detest the codition and skip mapping character 127. I
was not able to find more elegant way of detecting that t_kb is '\177',
but this should work too.
Restore cursor position when the buffer has been modified

Conque term uses basically three input modes. 'input' for entering
command, 'edit' one after pressing g:ConqueTerm_ToggleKey and 'normal'
one after pressing <Esc> (if g:ConqueTerm_EscKey is empty or set to
<Esc>. If you modify the buffer in 'normal' or 'edit' mode, cursor just
jumped back to it's original position without taking into account
whether there have been any lines added or removed above. This change
tries to fix that.

The mechanics are quite simple, if you return to 'input' mode, consult
mark '^ which holds where the 'input' mode was left. And marks are moved
automatically with the text bein edited. If we go to 'edit' mode, we can
actually enter and exit VIM insert mode several times before returning
back to Conque insert mode. So we need to save '^ somewhere - 'w was
chosen.

If we, during the 'normal' or 'edit' mode, delete the line containing
the returning mark (command line), we'll start new line at the bottom of
the file.