Auto merge of #2168 - mullvad:add-ip-bound-if, r=JohnTitor

Add IP_BOUND_IF for Apple

To allow for binding a socket to an interface or querying a socket to see which interface it's bound to, `IP_BOUND_IF` must be used - I've added it to `libc`.

I've added it as an apple-wide constant (both for macOS and iOS), but I'm not sure if it's the same value on iOS.

Author: bors

libc-test/semver/apple.txt

@@ -487,6 +487,7 @@ IPV6_RECVPKTINFO
 IPV6_RECVTCLASS
 IPV6_TCLASS
 IP_HDRINCL
+IP_BOUND_IF
 IP_PKTINFO
 IP_RECVDSTADDR
 IP_RECVIF

src/unix/bsd/apple/mod.rs

@@ -2432,6 +2432,7 @@ pub const IP_RECVDSTADDR: ::c_int = 7;
 pub const IP_ADD_MEMBERSHIP: ::c_int = 12;
 pub const IP_DROP_MEMBERSHIP: ::c_int = 13;
 pub const IP_RECVIF: ::c_int = 20;
+pub const IP_BOUND_IF: ::c_int = 25;
 pub const IP_PKTINFO: ::c_int = 26;
 pub const IP_RECVTOS: ::c_int = 27;
 pub const IPV6_JOIN_GROUP: ::c_int = 12;