Add fexecve cfg attribute for OpenBSD

fexecve doesn't exist on OpenBSD so add conditional compilation for it
in unistd and in related tests.

The only "reference" that I could find is a mention that fexecve is
not implemented on OpenBSD in the manual pages for signal(3) and
sigaction(2):

Official repository (search for "fexecve"):
https://cvsweb.openbsd.org/src/lib/libc/sys/sigaction.2?rev=1.75&content-type=text/x-cvsweb-markup

Github mirror:
https://github.com/openbsd/src/blob/master/lib/libc/sys/sigaction.2#L619

Author: thendiscard

src/unistd.rs

@@ -763,8 +763,7 @@ pub fn execvpe(filename: &CString, args: &[CString], env: &[CString]) -> Result<
 #[cfg(any(target_os = "android",
           target_os = "freebsd",
           target_os = "linux",
-          target_os = "netbsd",
-          target_os = "openbsd"))]
+          target_os = "netbsd"))]
 #[inline]
 pub fn fexecve(fd: RawFd, args: &[CString], env: &[CString]) -> Result<Void> {
     let args_p = to_exec_array(args);

test/test_unistd.rs

@@ -230,16 +230,16 @@ cfg_if!{
         execve_test_factory!(test_execve, execve, &CString::new("/system/bin/sh").unwrap());
         execve_test_factory!(test_fexecve, fexecve, File::open("/system/bin/sh").unwrap().into_raw_fd());
     } else if #[cfg(any(target_os = "freebsd",
-                        target_os = "linux",
-                        target_os = "openbsd"))] {
+                        target_os = "linux"))] {
         execve_test_factory!(test_execve, execve, &CString::new("/bin/sh").unwrap());
         execve_test_factory!(test_fexecve, fexecve, File::open("/bin/sh").unwrap().into_raw_fd());
     } else if #[cfg(any(target_os = "dragonfly",
                         target_os = "ios",
                         target_os = "macos",
-                        target_os = "netbsd"))] {
+                        target_os = "netbsd",
+                        target_os = "openbsd"))] {
         execve_test_factory!(test_execve, execve, &CString::new("/bin/sh").unwrap());
-        // No fexecve() on DragonFly, ios, macos, and NetBSD.
+        // No fexecve() on DragonFly, ios, macos, NetBSD, OpenBSD.
     }
 }