--interface: add support for Linux VRF

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