mksh @R50e_0: update to R50f

[Schamschula (Marius Schamschula)]

mksh has been updated to version R50f. New this version:

[tg] Add a patch marker for vendor patch versioning to mksh.1
[tg] SECURITY: make unset HISTFILE actually work
[tg] Document some more issues with the current history code
[tg] Remove some unused code
[tg] RCSID-only sync with OpenBSD, for bogus and irrelevant changes
[tg] Also disable field splitting for alias 'local=\typeset'
[tg] Fix read -n-1 to not be identical to read -N-1
[tg] Several fixes and improvements to lksh(1) and mksh(1) manpages
[tg] More code (int → size_t), comment and testsuite fixes
[tg] Make dot.mkshrc more robust (LP#1441853)
[tg] Fix issues with IFS='\' read, found by edualbus
[enh, tg] Fix integer overflows related to file descriptor parsing, found by Pawel Wylecial (LP#1440685); reduce memory usage for I/O redirs
[tg] Document in the manpage how to set ±U according to the current locale settings via LANG/LC_* parameters (cf. Debian #782225)
[igli, tg] Some code cleanup and restructuring
[tg, oksh] Handle number parsing and storing more carefully

[Ionic (Mihai Moldovan)]

Committed as r135240. Thanks!