File completion: fix incomplete multibyte support

Upon encountering two filenames with multibyte characters starting
with the same byte, a partial multibyte character was completed.

Reproducer (to run in UTF-8 locale):
$ touch XXXá XXXë
$ : XX		<== pres tab
$ : XXX^?	<== partial multibyte character appears

Note: á is $'\xc3\xa1' and ë is $'\xc3\xab' (same initial byte).

src/cmd/ksh93/edit/completion.c:
- Add multibyte support to the charcmp() and overlaid() functions.
  Thanks to Harald van Dijk for useful code and suggestions.
- Add a few missing mbinit() calls. The state of multibyte
  processing must be reset before starting a new loop in case a
  previous processing run was interrupted mid-character.

src/cmd/ksh93/tests/pty.sh:
- Add test based on Harald's reproducer.

Resolves: https://github.com/ksh93/ksh/issues/223

NEWS

@@ -3,6 +3,12 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-03-17:
+
+- Fixed a bug with file name completion on the interactive shell in multibyte
+  locales. Upon encountering two filenames with multibyte characters starting
+  with the same byte, a partial multibyte character was autocompleted.
+
 2021-03-16:
 
 - Tilde expansion can now be extended or modified by defining a .sh.tilde.get