breakcites – Ensure that multiple citations may break at line end

Makes a very mi­nor change to the op­er­a­tion of the \cite com­mand. Note that the change is not nec­es­sary in un­mod­i­fied LaTeX; how­ever, there re­main pack­ages that re­store the un­de­sir­able be­haviour of the com­mand as pro­vided in LaTeX 2.09. (Note that nei­ther cite nor nat­bib make this mis­take.)