urldata: convert bools to bitfields and move to end

urldata: convert bools to bitfields and move to end

This allows the compiler to pack and align the structs better in
memory. For a rather feature-complete build on x86_64 Linux, gcc 8.1.2
makes the Curl_easy struct 4.9% smaller. From 6312 bytes to 6000.

Removed an unused struct field.

No functionality changes.

Closes #3610