Ossec 2.8 prep

Removed old ossec 2.7 files and added basis of a doc to show how to prep the ossec files to be incuded in the securedrop-deb packages.
Notice the numerous ?'s in this copied text from the ossec install script. Given their placement within strings of otherwise readable characters in the context of foreign languages, this looks like an encoding bug. I talked to @dolanjs about this and confirmed that the issue occurs when running the ossec install script, and was not due to a copy-paste error. This appears to be a bug in OSSEC, or a conflict with OSSEC and some locale settings. We checked the locale on the system where we confirmed the issue and it was `en_US.UTF-8`, which is expected to work.

This is probably an OSSEC bug, just mentioning it here in case others wonder about it.