Log Message:
shells/pbosh: Update to 2020-05-25

- Use OS specific helper code from devel/smake/Makefile.common

Changes from AN-2020-05-25:
-       libmdgest: Added a "defined(_M_ARM)" check for MSVC byte order
        definitions (byte_order.h).

        Thanks to "Excalibur" for reporting.

-       Bourne Shell: The support for the OLDPWD environment has been moved
        from DO_SYSPUSHD to DO_POSIX_CD and as a result, the minimal POSIX
        compile variant "pbosh" now supports "$OLDPWD" and \ 
"cd -".

        This was based on a bug in the POSIX standard that did not include
        OLDPWD in the overview of shell environment variables. See
        https://www.austingroupbugs.net/view.php?id=1343

        Thanks to Koichi Nakashima for reporting