Make the Loader API thread-safe

Since the majority of the code is using ReadAt API already, map this to
a `readp` "syscall" which does not mutate any state about the file
descriptor therefore making it fairly safe multi-threading wise.

This allows to get rid of read-time mutexes in RamCachedFileLoader and
therefore fixes #9803

Core/FileLoaders/HTTPFileLoader.cpp

@@ -115,10 +115,6 @@ std::string HTTPFileLoader::Path() const {
 	return filename_;
 }
 
-void HTTPFileLoader::Seek(s64 absolutePos) {
-	filepos_ = absolutePos;
-}
-
 size_t HTTPFileLoader::ReadAt(s64 absolutePos, size_t bytes, void *data, Flags flags) {
 	Prepare();
 	s64 absoluteEnd = std::min(absolutePos + (s64)bytes, filesize_);