Why am I not allowed slashes in the proxy setting?
Your http://xxx.com/proxy.pac is not a proxy server, it is the address to a proxy automatic configuration (PAC) file. A PAC file contains a Javascript function that determines which proxy address to use and/or whether you're allowed to go direct when connecting to a particular URL.

ProxyDroid (requires root) claims to support proxy configuration using PAC file (I had never personally used it though).