zshenv: avoid placing empty components in PATH; reorder root's PATH

zshenv: avoid placing empty components in PATH; reorder root's PATH

zsh treats an empty PATH component ("::") as ".", which creates a
security hole. Sanitize PATH by explicitly removing empty
components.

As agreed on #grml, root's PATH should also list /usr/local/sbin
and /usr/local/bin before /sbin, /bin, /usr/sbin and /usr/bin;
and there is no harm in adding $HOME/bin unconditionally as the
first PATH component.