shell integration: make output more consistent

This cleans up the output for commands with shell support now that csh
and tcsh do not require SPACK_ROOT to be set. This also adds output
describing how to source the fish init script, and makes messages more
consistent across commands.

- [x] Remove SPACK_ROOT from csh/tcsh instructions
- [x] Add fish to shell instructions
- [x] make output consistent across commands