Upgrade to mksh R56c.

Upgrade to mksh R56c.

R56c is a bugfix-only release everyone must upgrade to:

[komh] Remove redundant OS/2-specific code, clean up others
[komh, tg] Fix drive-qualified (absolute and relative) DOS-style path support in realpath functionality, partially other places
[tg] Don’t substitute ${ENV:-~/.mkshrc} result again
[tg] Improve OS/2 $PATH (et al.) handling, drive-relative paths
[tg] Add MKSH_ENVDIR compile-time option for Jehanne and Plan 9
[tg] Limit nesting when parsing malformed code (Debian #878947)
[tg] Update wcwidth data with bugfixed script (still Unicode 10; resulting values are identical to glibc git master for extant chars)
[Dr. Werner Fink] Raise some time limits in the testsuite
[Shamar] Add support for the Jehanne operating system
[komh] Set stdin to text mode before executing child processes on OS/2
[komh] Pass arguments via a resonse file if executing a child fails
[Dr. Werner Fink] Early locale tracking as a compile-time option
[tg] Fix regressions introduced with new fast character classes

Bug: N/A
Test: builds and boots
Change-Id: I44da7a8bb9859a0357bcd59891dd4f2cfc199733