Standarized the use of brackets - double instead of single ones -
for test situations like if-clauses.
For info, see http://zsh.dotsrc.org/Guide/zshguide02.html#l8 , last
paragraph.
Furthermore several cases of 'if [[..]]; then` where changed to the
otherwise used form 'if [[..]] ; then`.
Oneliners used:
% sed -i 's,\]\;\( then\),\] ;\1,' etc/{skel,zsh}/.#z*
% sed -i 's,[^\["]\(\[ \+[-!"$]\+\), [\1,g;s,\( ]\)\([ ;]\+\),\1]\2,g' etc/{skel,zsh}/.#z*

I checked the result several times. I really hope I did not miss any
false positive.