Fixes to ossec-installer.nsi

Move the logic that determines whether the ossec.conf should be
replaced/renamed out of the C code and into NSIS. The NSIS stuff is
built for installing things. No need to write a bunch of C code to do
something that there is already a system for. Going to try and move
as much out of C and into NSIS to help cut down on the amount of code
that needs to be maintained for no real reason.