The --interface command (CURLOPT_INTERFACE option) already uses
SO_BINDTODEVICE on Linux, but it tries to parse it as an interface or IP
address first, which fails in case the user passes a VRF.
Try to use the socket option immediately and parse it as a fallback
instead. Update the documentation to mention this feature, and that it
requires the binary to be ran by root or with CAP_NET_RAW capabilities
for this to work.
Closes#2024
