Fix switching under leftright (FreeBSD bin/12801)
Do not break multibyte chars in ex_prchars.

But vi message output may still break them, especially UTF-8.
Bump version to 2.1.2 in README.
Sync with the OpenBSD manpage, which uses mdoc(7).
Display width 0 wide char with Unicode escape seq.

This change intends to make those characters editable and only
affects the vi editing area; ex messages and vi messages display
them as-is.

Closes-Bug: #10