Log Message:
Update mksh to R56b.

R56b is a bugfix-only release everyone should upgrade to:

- [tg] Reference the FAQ webpage
- [panpo, Riviera] Fix documentation bug wrt. Esc+Ctrl-L
- [tg, Larry Hynes] Fix '0' movement in vi mode
- [tg] Replace broken libcs' offsetof macro with MirBSD's

R56 is a bugfix release with some experimental fixes:

- [tg, Seb] Do not apply alias name restrictions to hash/tilde tracking
- [tg] Restore '.', ':' and '[' in alias names ('[[' is still forbidden)
- [tg] Fix accidentally defanged $PATHSEP test
- [tg] On ^C (INTR and QUIT edchars), shove edit line into history
- [iSKUNK, tg] Begin porting to z/OS using EBCDIC encoding, incomplete
- [tg] Redo fast character classes code, adding POSIX and other helpers
- [tg] bind parses backslash-escaped '^' (and '\') as escaped
- [tg] Building with -DMKSH_ASSUME_UTF8=0 no longer causes a known failure in
  the testsuite
- [tg] New test.sh option -U to pass a UTF-8 locale to use in the tests
- [tg] re_format(7) BSD: [[ $x = *[[:\<:]]foo[[:\>:]]* ]]
- [tg, iSKUNK] Use Config in check.pl only if it exists
- [tg] New matching code for bracket expressions, full POSIX (8bit)
- [komh] Exclude FAT/HPFS/NTFS-unsafe tests on OS/2 (and Cygwin/MSYS)
- [tg] Update to Unicode 10.0.0
- [tg, selk] Make readonly idempotent
- [tg, multiplexd] When truncating the persistent history, do not change the
  underlying file, do all operations on the locked one; do not stop using the
  history at all if it has been truncated
- [tg, J?rg] Turn off UTF-8 mode upon turning on POSIX mode
- [Martijn Dekker, Geoff Clare, many on the Austin list, tg] In POSIX mode,
  make the exec builtin force a $PATH search plus execve
- [tg] Fix GCC 7, Coverity Scan warnings
- [tg, Michal Hlavinka] Track background process PIDs even interactive
- [tg] Always expose mksh's hexdump shell function; speed it up by working on
  the input in chunks; use character classes to make it EBCDIC safe
- [tg] Revamp dot.mkshrc default editor selection mechanism