Auto merge of #2613 - djc:macos-dontfrag, r=Amanieu

apple IP{,V6}_DONTFRAG constants

These are new in macOS Big Sur, but I'm not sure if we're supposed to have any `cfg` incantations that can restrict to that version (I reviewed `cfg` instances in both `apple` and `linux_like` but didn't see anything like that).

Author: bors

src/unix/bsd/apple/mod.rs

@@ -3518,6 +3518,7 @@ 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 IP_DONTFRAG: ::c_int = 28;
 pub const IPV6_JOIN_GROUP: ::c_int = 12;
 pub const IPV6_LEAVE_GROUP: ::c_int = 13;
 pub const IPV6_CHECKSUM: ::c_int = 26;
@@ -3526,6 +3527,7 @@ pub const IPV6_TCLASS: ::c_int = 36;
 pub const IPV6_PKTINFO: ::c_int = 46;
 pub const IPV6_HOPLIMIT: ::c_int = 47;
 pub const IPV6_RECVPKTINFO: ::c_int = 61;
+pub const IPV6_DONTFRAG: ::c_int = 62;
 
 pub const TCP_NOPUSH: ::c_int = 4;
 pub const TCP_NOOPT: ::c_int = 8;