Update mksh to 40c.

* [tg, Wouter Verhelst] Fix ${foo%\?} in -o sh
* [jonthn, Snader_LB] Fix spelling mistakes
* [cnuke] Fix bugs spotted during porting to OPENSTEP
* [tg, jg71] Unbreak building stristr with MKSH_ASSUME_UTF8 defined
* [tg] Fix most of the issues Jerker Bäck encountered on Interix
* [tg] Wrap access(2) as it may return false positive for X_OK on root
* [OpenBSD] Upper bound Emacs mode command repeat by input line length
* [tg] Improve CPPFLAGS mangling in Build.sh
* [Snader_LB] Several comment, documentation and website fixes/updates
* [tg] Avoid identifiers and cpp(1) macros with two underscores in a row,
  and those with a trailing underscore; they are reserved for the OS
* [tg] Fix accidental behavioural change wrt some built-in parameters
* [RT, Chris “ir0nh34d” Sutcliffe] Port to MSYS
* [tg, Markus Duft, Bruno Haible] Interix select(2) bug workaround
* [tg] Manpage fixes
* [tg] Deprecate the “command line begins with ‘!’ for fc -e -” wbx hack
* [tg] Promote x=(a b); x+=(c d) to a core mksh(1) feature
* [tg] Testsuite fixes for Hurd, MSYS; warning fixes for dietlibc
* [tg] Fix R40 regression misparsing $(case x in (x) :; esac)

(bsiegert)