Fix build and tests issues on OpenBSD 6.4+

1)  lutimes doesn't exist on OpenBSD so it needs to be under conditional
    compilation.

    The only "reference" that I could find related to this is the discussion
    here: https://github.com/rust-lang/libc/pull/790 .

2)  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

3)  AIO doesn't work on OpenBSD so put test_aio_drop under conditional
    compilation.

4)  Add relevant changelog entries.

P.S. On OpenBSD remains the issue of test_scm_rights which builds
correctly but fails at runtime.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/unistd.rs b/src/unistd.rs
index 2c00f7f0..a41c1392 100644
--- a/src/unistd.rs
+++ b/src/unistd.rs
@@ -760,11 +760,12 @@ pub fn execvpe(filename: &CString, args: &[CString], env: &[CString]) -> Result<
 ///
 /// This function is similar to `execve`, except that the program to be executed
 /// is referenced as a file descriptor instead of a path.
+// Note for NetBSD and OpenBSD: although rust-lang/libc includes it (under
+// unix/bsd/netbsdlike/) fexecve is not currently implemented on NetBSD nor on
+// OpenBSD.
 #[cfg(any(target_os = "android",
-          target_os = "freebsd",
           target_os = "linux",
-          target_os = "netbsd",
-          target_os = "openbsd"))]
+          target_os = "freebsd"))]
 #[inline]
 pub fn fexecve(fd: RawFd, args: &[CString], env: &[CString]) -> Result<Void> {
     let args_p = to_exec_array(args);