Expand config search paths #537

Moving some review items from this pull request into a new issue:
#534

That pull request adds some basic user-directory search pathing for the config file. This issue expands the search paths on different platforms:

There are more places worth checking, too.

On Linux:

• $XDG_CONFIG_HOME/osvr/osvr_server_config.json
• For each colon-separated dir in $XDG_CONFIG_DIRS: $prefix/osvr/osvr_server_config.json
• /etc/xdg if $XDG_CONFIG_DIRS is not set or empty.
• /etc/osvr_server_config.json for old-school paths
• /usr/local/etc/osvr_server_config.json for old-school paths

On macOS:

• $HOME/Library/Application Support/OSVR/config/osvr_server_config.json
• /Library/Application Support/OSVR/config/osvr_server_config.json
• wherever brew installs it by default (I think /usr/local/Cellar/osvr-core/bin/osvr_server_config.json, maybe)
• /etc/osvr_server_config.json for old-school paths
• /usr/local/etc/osvr_server_config.json for old-school paths