Update mksh to R57.

R57 rolls up bugfixes, with few hard changes:

  * [gecko2] Update operating environment reporting for the Macintosh
  * [Martijn Dekker] make ${foo#'bar'} in here document behave like ksh93
  * [Martijn Dekker] quote empty strings for re-entry into shell
  * [tg, G.raud Meyer] Improve documentation, especially for tty states
  * [tg] Protect against entering line editing with bad saved tty state
  * [tg] Fix set -o allexport for arrays (which we apparently do)
  * [tg] Handle lseek(2) returning -1 as pointed out by Coverity Scan
  * [tg] Fix left-padding UTF-8 strings
  * [tg, G.raud Meyer] Fix using the ?-m? flag on the command line
  * [tg] Update to UCD 11.0.0
  * [multiplexd] Fix a segfault using ^W during search in Vi mode
  * [tg] Fix an error message; add a test for controlling tty
  * [tg] Permit unsetting LINES and COLUMNS, for those who need it
  * [tg] Fix manpage bug (RedHat BZ#1612173)
  * [tg] Minor spelling cleanup
  * [tg] Unbreak high-bit7 (n?n-ASCII) heredoc separators (LP#1779179)
  * [tg] Allow dumping high-bit7-char-containing strings in DEBUG mode
  * [tg] Add some testcases for behaviour questions popped up in IRC
  * [tg] Trick a GCC warning, to make up for it ignoring lint(1) hints
  * [tg] Add O_MAYEXEC support for CLIP OS
  * [tg] Make dup-to-self with ksh-style fd?3 closing work; catern via IRC
  * [tg] Add compat glue for newer GNU groff mdoc to the manpages
  * [tg] Trigger EXIT trap after single-command subshells (Debian #910276)
  * [tg] Document set -eo pipefail caveat (LP#1804504)
  * [tg] Fix MKSH_EARLY_LOCALE_TRACKING warning
  * [tg] Document that, when your Unix is broken, GIGO applies (LP#1817959)
  * [tg] Improve error message for inaccessible executables (LP#1817789)

(bsiegert)