Rebooting 93u+m

This commit reboots ksh 93u+m development with a new fork based off
the archived official AT&T ast repo, master branch.

README:
- Removed. Having two READMEs is a botch.

README.md:
- Imported and edited from the archived fork,
  https://github.com/modernish/ksh -- the newer instructions will
  become applicable as I cherry-pick all the changes from there :)
    - Copy-edits.
    - Added policy and rationale for this 93u+m fork.
    - Incorporated info from old README that is still relevant.

.gitignore:
- Added.

bin/execrate:
- Removed. This one keeps changing its license header, causing git
  to show an uncommitted file. It's always re-copied anyway, and
  not for direct invocation, so, exclude and add to .gitignore.
  The source file is: src/cmd/INIT/execrate.sh

src/cmd/*,
src/lib/*:
- Remove historical baggage: everything except ksh93 and
  dependencies. This is the same stuff the ksh-community fork
  removed. I wouldn't mind keeping it, but some of this stuff
  causes build failures on macOS (and probably other systems), and
  I need to get on with fixing ksh93.
     Hopefully, we can figure out how to re-add stuff we can use
  later -- particularly the pty command (pseudo-terminal utility
  for scripting interactive sessions) which is needed for
  regression-testing the interactive shell.

bin/package

@@ -1,4 +1,4 @@
-USAGE_LICENSE="[-author?Glenn Fowler <gsf@research.att.com>][-copyright?Copyright (c) 1994-2020 AT&T Intellectual Property][-license?http://www.eclipse.org/org/documents/epl-v10.html]"
+USAGE_LICENSE="[-author?Glenn Fowler <gsf@research.att.com>][-copyright?Copyright (c) 1994-2012 AT&T Intellectual Property][-license?http://www.eclipse.org/org/documents/epl-v10.html]"
 ########################################################################
 #                                                                      #
 #               This software is part of the ast package               #