Revert "style: remove _MAX* place holder constants"

This reverts commit 0f93088.

`RTAX_MAX` is used to loop over alternate internal encoding.

Author: tesuji

src/unix/bsd/freebsdlike/dragonfly/mod.rs

@@ -1533,6 +1533,7 @@ pub const RTM_VERSION: ::c_int = 7;
 pub const RTAX_MPLS1: ::c_int = 8;
 pub const RTAX_MPLS2: ::c_int = 9;
 pub const RTAX_MPLS3: ::c_int = 10;
+pub const RTAX_MAX: ::c_int = 11;
 
 const_fn! {
     {const} fn _CMSG_ALIGN(n: usize) -> usize {

src/unix/bsd/freebsdlike/freebsd/mod.rs

@@ -4561,6 +4561,8 @@ pub const RTF_FIXEDMTU: ::c_int = 0x80000;
 
 pub const RTM_VERSION: ::c_int = 5;
 
+pub const RTAX_MAX: ::c_int = 8;
+
 // sys/signal.h
 pub const SIGTHR: ::c_int = 32;
 pub const SIGLWP: ::c_int = SIGTHR;

src/unix/bsd/netbsdlike/netbsd/mod.rs

@@ -2439,6 +2439,7 @@ pub const RTM_CHGADDR: ::c_int = 0x18;
 pub const RTA_TAG: ::c_int = 0x100;
 
 pub const RTAX_TAG: ::c_int = 8;
+pub const RTAX_MAX: ::c_int = 9;
 
 cfg_if! {
 

src/unix/bsd/netbsdlike/openbsd/mod.rs

@@ -1962,6 +1962,7 @@ pub const RTAX_BFD: ::c_int = 11;
 pub const RTAX_DNS: ::c_int = 12;
 pub const RTAX_STATIC: ::c_int = 13;
 pub const RTAX_SEARCH: ::c_int = 14;
+pub const RTAX_MAX: ::c_int = 15;
 
 const_fn! {
     {const} fn _ALIGN(p: usize) -> usize {