memoize visualize_wchar results.
fixed inserting a new line on line-down in text mode.
don't include legacy tables for ancient terminals.
	* block.cc, edit.h: get rid of DosEol flag; add support for MAC EOL (single CR).
	* ext.cc: (DOS_UNIX) improve performance.
	* kern.cc: MAC EOL support; check for split MB chars after insert/delete;
	  improve performance of EolAt/BolAt;
	* keymap.cc: buffer incomplete MB chars and feed them at once.
	* loadsave.cc: guess MAC EOLs.
	* mb.cc, mb.h: improve performance of MBCheckLeftAt; (MBCharSplit) new flag;
	* screen.cc: display MAC EOL mode as 'M'.
	* textpoin.cc, textpoin.h: (CheckSplit) new function; more precise
	  tracking of split MB chars; improved FindLineCol performance.
fixed DOS EOL guessing