define `sethostname` under OpenBSD and Bitrig

factorize the definition with NetBSD

src/unix/bsd/netbsdlike/mod.rs

@@ -529,6 +529,7 @@ extern {
                         abstime: *const ::timespec) -> ::c_int;
    pub fn pthread_condattr_setclock(attr: *mut pthread_condattr_t,
                                     clock_id: clockid_t) -> ::c_int;
+    pub fn sethostname(name: *const ::c_char, len: ::size_t) -> ::c_int;
 }
 
 cfg_if! {

src/unix/bsd/netbsdlike/netbsd.rs

@@ -588,7 +588,6 @@ extern {
                   pid: ::pid_t,
                   addr: *mut ::c_void,
                   data: ::c_int) -> ::c_int;
-    pub fn sethostname(name: *const ::c_char, len: ::size_t) -> ::c_int;
     pub fn pthread_setname_np(t: ::pthread_t,
                               name: *const ::c_char,
                               arg: *mut ::c_void) -> ::c_int;