Use sbbs.ini [web] FileVPathPrefix to configure web filebase prefix

It bothered Deuce having a web server setting in scfg_t/SCFG->File Options, so I moved this setting to [web_]startup_t and the sbbs.ini file.

The downside is that file_area.web_file_prefix is no longer available to JS environments outside of the web server and terminal server, but meh, probably not going to use it elsewhere anyway? I can imagine use cases for JSexec scripts to want to generate URLs to filebase files. If that ends up being a need, they'll have to find and parse the "right" sbbs.ini file to determine the vpath prefix.