Singular matrix warning IDs

[John W. Eaton]

I checked in the following change so that warning IDs are used for all 
cases where Octave warns about singular matrices when inverting matrices 
or solving systems of equations.

http://hg.savannah.gnu.org/hgweb/octave/rev/a0c7001cf1a8

Note that in some cases the singular matrix warning is followed by 
another warning message about attempting to find a minimum norm 
solution.  I'm not sure whether that warning should be controlled by a 
separate ID, the same ID as for the singular matrix warning, or simply 
removed.

I'm leaning toward just removing it.  Any objections?

jwe