Auto merge of #2178 - williewillus:openbsd-unveil, r=JohnTitor

Add OpenBSD unveil(2)

https://man.openbsd.org/unveil

Author: bors

libc-test/semver/openbsd.txt

@@ -1071,6 +1071,7 @@ ttyname_r
 udf_args
 ufs_args
 unmount
+unveil
 useconds_t
 uselocale
 utimensat

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

@@ -1465,6 +1465,7 @@ extern "C" {
         envp: *const *const ::c_char,
     ) -> ::c_int;
     pub fn pledge(promises: *const ::c_char, execpromises: *const ::c_char) -> ::c_int;
+    pub fn unveil(path: *const ::c_char, permissions: *const ::c_char) -> ::c_int;
     pub fn strtonum(
         nptr: *const ::c_char,
         minval: ::c_longlong,