Added sbbs_t::getstr_offset member variable, used with getstr(K_USEOFFSET)

for full-screen editor functionality.
Added support for CON_DOWNARROW, CON_LEFTARROW, and CON_BACKSPACE console
status bits, used in combination with getstr(K_LEFTEXIT)  for full-screen
editor functionality.
Insert mode (^V) still needs work.