openbsd: constantify some arguments of openpty() and forkpty()

OpenBSD recently made termp and winp arguments of openpty() and forkpty() const. 
to match the prototypes in glibc and musl libc.

src/unix/bsd/netbsdlike/mod.rs

@@ -688,19 +688,6 @@ extern "C" {
         flag: ::c_int,
     ) -> ::c_int;
     pub fn fdatasync(fd: ::c_int) -> ::c_int;
-    pub fn openpty(
-        amaster: *mut ::c_int,
-        aslave: *mut ::c_int,
-        name: *mut ::c_char,
-        termp: *mut termios,
-        winp: *mut ::winsize,
-    ) -> ::c_int;
-    pub fn forkpty(
-        amaster: *mut ::c_int,
-        name: *mut ::c_char,
-        termp: *mut termios,
-        winp: *mut ::winsize,
-    ) -> ::pid_t;
     pub fn login_tty(fd: ::c_int) -> ::c_int;
     pub fn getpriority(which: ::c_int, who: ::id_t) -> ::c_int;
     pub fn setpriority(which: ::c_int, who: ::id_t, prio: ::c_int) -> ::c_int;

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

@@ -2315,6 +2315,20 @@ extern "C" {
         attrnamespace: *mut ::c_int,
     ) -> ::c_int;
 
+    pub fn openpty(
+        amaster: *mut ::c_int,
+        aslave: *mut ::c_int,
+        name: *mut ::c_char,
+        termp: *mut ::termios,
+        winp: *mut ::winsize,
+    ) -> ::c_int;
+    pub fn forkpty(
+        amaster: *mut ::c_int,
+        name: *mut ::c_char,
+        termp: *mut ::termios,
+        winp: *mut ::winsize,
+    ) -> ::pid_t;
+
     #[link_name = "__lutimes50"]
     pub fn lutimes(file: *const ::c_char, times: *const ::timeval) -> ::c_int;
     #[link_name = "__gettimeofday50"]

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

@@ -1644,6 +1644,20 @@ extern "C" {
     pub fn pthread_set_name_np(tid: ::pthread_t, name: *const ::c_char);
     pub fn pthread_stackseg_np(thread: ::pthread_t, sinfo: *mut ::stack_t) -> ::c_int;
 
+    pub fn openpty(
+        amaster: *mut ::c_int,
+        aslave: *mut ::c_int,
+        name: *mut ::c_char,
+        termp: *const ::termios,
+        winp: *const ::winsize,
+    ) -> ::c_int;
+    pub fn forkpty(
+        amaster: *mut ::c_int,
+        name: *mut ::c_char,
+        termp: *const ::termios,
+        winp: *const ::winsize,
+    ) -> ::pid_t;
+
     pub fn sysctl(
         name: *const ::c_int,
         namelen: ::c_uint,