This removes the StrCaseCmp() stuff from 'net idmap' and 'net

This removes the StrCaseCmp() stuff from 'net idmap' and 'net
groupmap'.  The correct way to implement this stuff is via a function
table, as exampled in all the other parts of 'net'.

This also moves the idmap code into a new file.  Volker, is this your
code?  You might want to put your name on it.

Andrew Bartlett
(This used to be commit 477f2d9e390bb18d4f08d1cac9c981b73d628c4f)