Update mksh to R50b, prodded by tg.

Changelog:
R50b is a recommended bugfix release:

* [Ypnose] Fix operator description in the manpage
* [tg] Change all mention of “eglibc” to “glibc”, it is merged back
* [Colona] Fix rare infinite loop with invalid UTF-8 in the edit buffer
* [tg] Make more clear when a shell is interactive in the manpage
* [tg] Document that % is a symmetric remainder operation, and how to
  get a mathematical modulus from it, in the manpage
* [tg, Christopher Ferris, Elliott Hughes] Make the cat(1) builtin also
  interruptible in the write loop, not just in the read loop, and avoid
  it getting SIGPIPE in the smores function in dot.mkshrc by terminating
  cat upon user quit
* [tg] Make some comments match the code, after jaredy from obsd changed
  IFS split handling
* [tg] Fix some IFS-related mistakes in the manual page
* [tg] Document another issue as known-to-fail test IFS-subst-3
* [tg] Improve check.pl output in some cases
* [tg, Jb_boin] Relax overzealous nameref RHS checks

R50 is a recommended bugfix release:

* [tg] Fix initial IFS whitespace not being ignored when expanding
* [tg] MKSH_BINSHREDUCED no longer mistakenly enables brace expansion
* [tg] Explain more clearly Vi input mode limitations in the manpage
* [tg] Improve error reporting of the check.pl script (which needs a
  maintainer since I don’t speak any perl(1), really), for lewellyn
* [tg] Use $TMPDIR in test.sh for scratch space
* [tg, Polynomial-C] Check that the scratch space is not mounted noexec
* [pekster, jilles, tg] Use termcap(5) names, not terminfo(5) names, in
  tput(1) examples, for improved portability (e.g. to MidnightBSD)
* [tg] Avoid C99 Undefined Behaviour in mirtoconf LFS test (inspired by
  Debian #742780)
* [tg] Fix ${!foo} for when foo is unset
* [tg] Improve nameref error checking (LP#1277691)
* [tg] Fix readonly bypass found by Bert Münnich
* [Ryan Schmidt] Improved system reporting for Mac OS X
* [nDuff] Explain better [[ extglob handling in the manpage
* [tg] Remove arr=([index]=value) syntax due to regressions
* [tg] IFS-split arithmetic expansions as per POSIX 201x
* [OpenBSD] Add more detailed Authors section to manpage
* [tg] Fix set ±p issue for good: drop privs unless requested
* [tg] Improve signal handling and use a more canonical probing order
* [tg] Fix return values $? and ${PIPESTATUS[*]} interaction with set -o
  pipefail and COMSUBs
* [enh] Detect ENOEXEC ELF files and use a less confusing error message
* [tg] Update to Unicode 7.0.0
* [tg] Shut up valgrind in the $RANDOM code
* [tg] Use -fstack-protector-strong in favour of -fstack-protector-all
* [tg] Fix access-after-free crash spotted by Enjolras via IRC


(bsiegert)