Fix replace clobbering head of line

Regression introduced in 7f44bdde01411db1ccc73eb512694c3798c1ce4b
Fix replace action when cursor is before word

`<cword>` returns the next word when the cursor is on non-
word characters.
Make replace operation multi-byte safe
Set cursor on end of inserted word after replace
Add more loading guards

* Compatible mode
* unite function not found
Abort replace action on error