mig wrapper to kmig/emig

mig wrapper to kmig/emig

It won't handle first time setup; you'll need to call emig or kmig explicitly
until an environment is loaded.

Example:

    kmig env create demo
    kmig env use demo
    mig init
    mig add foo.txt etc

Also, the older ~/.mig environment directory, if used with the new mig, will
need to be moved or merged into ~/.emig, and the various *.env files ammended
with:

    export MIGCMD=emig