Use file command in ossec-installer.nsi

Use the full ability of the the File command. Before when upgrading and doing a Rename after without the /Reboot command most of those commands would "fail silently" which is the best way I can describe it. It would just leave these files in the main ossec-agent directory never really upgrading parts of the system. Using the File command has the added benefit of complaining if a file is in use during the installation. For example have the win32ui.exe open and try to run a new installation. It hould complain that the file is inaccessible until the application is closed. Previously, this would just leave os_win32.exe in the ossec-agent directory and never successfully upgrade the executable.
Merge pull request #126 from awiddersheim/patch-7

Use file command in ossec-installer.nsi