Fix corrupt UTF-8 char processing & shellquoting after aborted read

If the processing of a multibyte character was interrupted in UTF-8
locales, e.g. by reading just one byte of a two-byte character 'ü'
(\303\274) with a command like:
	print -nr $'\303\274' | read -n1 g
then the shellquoting algorithm was corrupted in such a way that
the final quote in simple single-quoted string was missing. This
bug may have had other, as yet undiscovered, effects as well. The
problem was with corrupted multibyte character processing and not
with the shell-quoting routine sh_fmtq() itself.

Full trace and discussion at: https://github.com/ksh93/ksh/issues/5
(which is also an attempt to begin to understand the esoteric
workings of the libast mb* macros that process UTF-8 characters).

src/lib/libast/comp/setlocale.c: utf8_mbtowc():
- If called from the mbinit() macro (i.e. if both pointer
  parameters are null), reset the global multibyte character
  synchronisation state variable. This fixes the problem with
  interrupted processing leaving an inconsistent state, provided
  that mbinit() is called before processing multibyte characters
  (which it is, in most (?) places that do this). Before this fix,
  calling mbinit() in UTF-8 locales was a no-op.

src/cmd/ksh93/sh/string.c: sh_fmtq():
- Call mbinit() before potentially processing multibyte characters.
  Testing suggests that this could be superfluous, but at worst,
  it's harmless; better be sure.

src/cmd/ksh93/tests/builtins.sh:
- Add regression test for shellquoting with 'printf %q' after
  interrupting the processing of a multibyte characeter with
  'read -n1'. This test only fails in a UTF-8 locale, e.g. when
  running: bin/shtests -u builtins SHELL=/buggy/ksh-2012-08-01

Fixes #5.

src/cmd/ksh93/tests/builtins.sh

@@ -302,7 +302,23 @@ fi
 if	[[ $(printf '%..*s\n' : abc def) != abc:def ]]
 then	err_exit "printf '%..*s' not working"
 fi
+
+# ======
+# shell-quoting using printf %q (same algorithm used for xtrace and output of 'set', 'trap', ...)
+
 [[ $(printf '%q\n') == '' ]] || err_exit 'printf "%q" with missing arguments'
+
+# the following fails on 2012-08-01 in UTF-8 locales
+expect="'shell-quoted string'"
+actual=$(
+	print -nr $'\303\274' | read -n1 foo  # interrupt processing of 2-byte UTF-8 char after reading 1 byte
+	printf '%q\n' "shell-quoted string"
+)
+LC_CTYPE=POSIX true	    # on buggy ksh, a locale re-init via temp assignment restores correct shellquoting
+[[ $actual == "$expect" ]] || err_exit 'shell-quoting corrupted after interrupted processing of UTF-8 char' \
+				"(expected $expect; got $actual)"
+
+# ======
 # we won't get hit by the one second boundary twice, right?
 expect= actual=
 { expect=$(LC_ALL=C date | sed 's/ GMT / UTC /') && actual=$(LC_ALL=C printf '%T\n' now) && [[ $actual == "$expect" ]]; } ||