Upgrade to mksh R57.

Upgrade to mksh 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 (nōn-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)

Test: treehugger
Change-Id: Ib5447fc474e509698bd04cdf70ed14b54e84d27a