add mksh package (#3375)

* add mksh package

adds MirBSD Korn Shell (v R57) package

* fix mksh package dependencies

removes make and manpages as these are already
chromebrew core dependencies

* add mksh postinstall message

warns user that mksh cannot be set as the default shell
and informs them of the example .mkshrc found in
../share/doc/mksh/examples

* modify mksh post-install message slightly

slight wording change when letting user know
about example .mkshrc file they can copy into
home directory

packages/mksh.rb

+require 'package'
+
+class Mksh < Package
+  description 'The MirBSD Korn Shell'
+  homepage 'https://www.mirbsd.org/mksh.htm'
+  version '0.57'
+  source_url 'http://www.mirbsd.org/MirOS/dist/mir/mksh/mksh-R57.tgz'
+  source_sha256 '3d101154182d52ae54ef26e1360c95bc89c929d28859d378cc1c84f3439dbe75'
+
+  def self.build
+    system 'sh Build.sh'
+  end
+
+  def self.check
+    system 'sh test.sh'
+  end
+
+  def self.install
+    FileUtils.mkdir_p "#{CREW_DEST_PREFIX}/bin"
+    system "install -c -s -m 555 mksh #{CREW_DEST_PREFIX}/bin/mksh"
+#    Can't perform this step unless the filesystem is mounted as RW:
+#    system 'grep -x /bin/mksh /etc/shells >/dev/null || echo /bin/mksh >>/etc/shells'
+    FileUtils.mkdir_p "#{CREW_DEST_PREFIX}/share/doc/mksh/examples"
+    system "install -c -m 444 dot.mkshrc #{CREW_DEST_PREFIX}/share/doc/mksh/examples/"
+    FileUtils.mkdir_p "#{CREW_DEST_PREFIX}/share/man/man1"
+    system "install -c -m 444 lksh.1 mksh.1 #{CREW_DEST_PREFIX}/share/man/man1/"
+  end
+
+  def self.postinstall
+    puts 'Please note: mksh cannot be set as the default shell in Chrome OS, as by default /etc' .lightgreen
+    puts 'is mounted as read-only, so mksh cannot be added to the list of valid login shells in /etc/shells.' .lightgreen
+    puts "For an example ~/.mkshrc file, copy #{CREW_PREFIX}/share/doc/mksh/examples/dot.mkshrc to #{HOME}/.mkshrc" .lightblue
+  end
+end