* 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