We now store the response headers in a download object on the protocol
server side and pass it to the client when finishing up a download.
Response headers are passed as an IPC::Dictionary. :^)
`ResourceLoader::load` now rejects URLs which specify a `port`
associated with network services known to be vulnerable to
inter-protocol exploitation.
Fixes#1735
StartDownload requests for unhandled protocols (or invalid URLs) will
now refuse to load instead of asserting. A failure code is sent back
to LibProtocol and Protocol::Client::start_download() returns nullptr.
Fixes#1604.
This function creates a nested event loop and runs a load() operation
inside it, returning only once the load has either succeeded or failed.
This will be used to implement blocking loads (ew!)
