Testing mksh as bootstrap shell

[Jonathan Perkin <jperkin%joyent.com@localhost>]

Hey,

As per recent discussions, we should start looking at alternatives to
shells/pdksh as our only available bootstrap shell, as it has bugs and
nobody has been interested in maintaining it.

A number of years ago I implemented shells/dash as a bootstrap option:

  https://github.com/joyent/pkgsrc-legacy/commit/61734467

and initially had a look at updating that to a newer release, but
quickly ran into build issues on macOS.  It appears that a number of
patches were still required from my prior attempt 6 years ago too.

So, on Benny's recommendation, I took a look at shells/mksh instead,
and it actually looks pretty good.  Not much larger than dash, looks
very portable, and so far has only needed one small patch to fix an
issue with bootstrapping using the legacy Solaris /bin/sh.

I have a branch here:

  https://github.com/joyent/pkgsrc/tree/joyent/dev/bootstrap-mksh

or you can just cherry-pick the commits here:

  https://github.com/joyent/pkgsrc/commit/b3167a64
  https://github.com/joyent/pkgsrc/commit/ed3a4c8b

I'm currently testing this in a full bulk build on my
macos15-trunk-x86_64 build hosts, and have successfully bootstrap on
my Solaris 9/x86 VM, but would appreciate some other platform tests,
as well as opinions on other colours we should be painting this
bikeshed instead.

Thanks,

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com