mksh R56c

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