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

src/cmd/ksh93/tests/pty.sh

@@ -772,5 +772,18 @@ w echo $? ~\t
 u 42 /tmp
 !
 
+# err_exit #
+((SHOPT_MULTIBYTE)) &&
+[[ ${LC_ALL:-${LC_CTYPE:-${LANG:-}}} =~ [Uu][Tt][Ff]-?8 ]] &&
+touch $'XXX\xc3\xa1' $'XXX\xc3\xab' &&
+tst $LINENO <<"!"
+L autocomplete should not fill partial multibyte characters
+# https://github.com/ksh93/ksh/issues/223
+d 15
+p :test-1:
+w : XX\t
+r ^:test-1: : XXX\r\n$
+!
+
 # ======
 exit $((Errors<125?Errors:125))