Auto merge of #3450 - larseggert:feat-ttl, r=JohnTitor

Add `IP_RECVTTL` and `IPV6_RECVHOPLIMIT` for Apple platforms

This adds `IP_RECVTTL` and `IPV6_RECVHOPLIMIT` for Apple platforms, which are currently missing.

(There are several other socket options present in `netinet/in.h` and `netinet6/in6.h`. I didn't add these since my code won't need them. Should I add them anyway?)

Author: bors

libc-test/semver/apple.txt

@@ -640,6 +640,7 @@ IPV6_HOPLIMIT
 IPV6_JOIN_GROUP
 IPV6_LEAVE_GROUP
 IPV6_PKTINFO
+IPV6_RECVHOPLIMIT
 IPV6_RECVPKTINFO
 IPV6_RECVTCLASS
 IPV6_TCLASS
@@ -649,6 +650,7 @@ IP_PKTINFO
 IP_RECVDSTADDR
 IP_RECVIF
 IP_RECVTOS
+IP_RECVTTL
 IP_TOS
 ITIMER_PROF
 ITIMER_REAL

src/unix/bsd/apple/mod.rs

@@ -4075,6 +4075,7 @@ 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_RECVTTL: ::c_int = 24;
 pub const IP_BOUND_IF: ::c_int = 25;
 pub const IP_PKTINFO: ::c_int = 26;
 pub const IP_RECVTOS: ::c_int = 27;
@@ -4084,6 +4085,7 @@ pub const IPV6_LEAVE_GROUP: ::c_int = 13;
 pub const IPV6_CHECKSUM: ::c_int = 26;
 pub const IPV6_RECVTCLASS: ::c_int = 35;
 pub const IPV6_TCLASS: ::c_int = 36;
+pub const IPV6_RECVHOPLIMIT: ::c_int = 37;
 pub const IPV6_PKTINFO: ::c_int = 46;
 pub const IPV6_HOPLIMIT: ::c_int = 47;
 pub const IPV6_RECVPKTINFO: ::c_int = 61;