for mksh but not lksh; bump to R51-alpha.
While here, tweak build scripts a bit, fixup MirBSD-specific Makefile
things, remove part of a comment that’s uninteresting.
(as, on some systems and for some pedants, it may be *better* suited
due to its broken use of the host C “long” data type for arithmetics)
but warn that this can only work with -DMKSH_BINSHPOSIX really
command; setting one still unsets the other at first)
• Change subst_exstat to be conformant unless -o sh is set and -o posix isn’t
• In lksh, make subst_exstat (newly) conformant if -o posix
• New MKSH_BINSHPOSIX to accompany MKSH_BINSHREDUCED
• Sync lksh manpage precisely
• make parsing numbers with leading digit-zero as octal independent of
mksh/lksh and dependent on set -o posix; adjust manpages to match
• warn about these changes and why mksh uses 32-bit consistent arithmetics
and point people to lksh for host-long undefined-behaviour arithmetics
• point out, explicitly, that it is *legal* for the operating environment
to make 'print $((2147483647 + 1))' (on a 32-bit system; adjust for a
64-bit system) to run 'rm -rf ~ /' instead
