Sync with FreeBSD:
* Change semantics of -i (in-place editing) so that it treats each file
independently from other files (desired in most practical cases). Keep
the previous semantics (use a single continuous address space covering
all files to edit in-place) under -I.
* Add -l option (make output line buffered).
* Make the 'y' command (translate) aware of multibyte characters.
* Add case-insensitive matching, using the 'I' flag, a la GNU sed.
* Remove 3rd clause.
* Raise WARNS to 6.
* Miscellaneous fixes and style(9) issues.