macos: add TCP_FASTOPEN_FORCE_ENABLE

This is used to disable the backoff mechanism, otherwise TFO is basically unusable.

Updates #1632 and #1635

Definition: https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/bsd/netinet/tcp.h#L310

Related commits/PRs: zonyitoo/tokio-tfo#5, database64128/tfo-go@c980f6b

Author: database64128

libc-test/semver/apple.txt

@@ -1349,6 +1349,7 @@ TAB2
 TAB3
 TABDLY
 TCP_FASTOPEN
+TCP_FASTOPEN_FORCE_ENABLE
 TCP_KEEPALIVE
 TCP_KEEPCNT
 TCP_KEEPINTVL

src/unix/bsd/apple/mod.rs

@@ -3657,6 +3657,8 @@ pub const TCP_KEEPINTVL: ::c_int = 0x101;
 pub const TCP_KEEPCNT: ::c_int = 0x102;
 /// Enable/Disable TCP Fastopen on this socket
 pub const TCP_FASTOPEN: ::c_int = 0x105;
+/// Disable/Enable TCP Fastopen backoff mechanism.
+pub const TCP_FASTOPEN_FORCE_ENABLE: ::c_int = 0x218;
 
 pub const SOL_LOCAL: ::c_int = 0;