Add a more useful solution to the problem of password files with more than

Add a more useful solution to the problem of password files with more than
one user who differs only by case. The other perl tools assume (or enforce)
the all lowercase requirement, therefore making the search through
master.passwd case insensitive seemed a reasonable optimization, IMO.

I understand, although I do not sympathize with, the argument that someone
might want to do this on purpose, and might subsequently want to use the
wrong tool for the job. So, this fix should hopefully satisfy both camps.