Work on issue #101 to remove references to $MBS_UNAME

We currently have mbs-get-user, but it is not being used. Packages should be updated to use mbs-get-user instead of $USER in case
we ever support other shells. For now, it will just sit there.

modified:   core/mbs-lib.sh
modified:   core/mbs.sh
modified:   ext/mbs-env.sh