- suppress error messages that appear with zsh.
According to unset(1p), 'unset -f' unsets a function.
If the function was not previously defined, this is a no-op.
This is similar for zsh, but there it emits a warning if the
function was not previously defined.
- avoid 'local' in functions sources from etc/bashrc.
ksh does not support this.
- use 'command' shell builtin instead of 'type'.
Seems to be more consistent between shell flavours.
