socket: support binding to interface *AND* IP

Introduce new notation for CURLOPT_INTERFACE / --interface:
ifhost!<interface>!<host>

Binding to an interface doesn't set the address, and an interface can
have multiple addresses.

When binding to an address (without interface), the kernel is free to
choose the route, and it can route through any device that can access
the target address, not necessarily the one with the chosen address.

Moreover, it is possible for different interfaces to have the same IP
address, on which case we need to provide a way to be more specific.

Factor out the parsing part of interface option, and add unit tests:
1663.

Closes #13719

lib/cf-socket.h

@@ -54,6 +54,11 @@ struct Curl_sockaddr_ex {
 };
 #define sa_addr _sa_ex_u.addr
 
+/*
+ * Parse interface option, and return the interface name and the host part.
+*/
+CURLcode Curl_parse_interface(const char *input, size_t len,
+                              char **dev, char **iface, char **host);
 
 /*
  * Create a socket based on info from 'conn' and 'ai'.