Auto merge of rust-lang#709 - wezm:setresuid, r=alexcrichton

Add setresgid & setresuid to FreeBSD and OpenBSD

These functions are both present in [FreeBSD](https://www.freebsd.org/cgi/man.cgi?query=setresuid&apropos=0&sektion=0&manpath=FreeBSD+11.1-RELEASE+and+Ports&arch=default&format=html) and [OpenBSD](http://man.openbsd.org/setresuid).

Author: bors

src/unix/bsd/freebsdlike/mod.rs

@@ -955,6 +955,8 @@ extern {
     pub fn getutxline(ut: *const utmpx) -> *mut utmpx;
     pub fn pututxline(ut: *const utmpx) -> *mut utmpx;
     pub fn setutxent();
+    pub fn setresgid(rgid: ::gid_t, egid: ::gid_t, sgid: ::gid_t) -> ::c_int;
+    pub fn setresuid(ruid: ::uid_t, euid: ::uid_t, suid: ::uid_t) -> ::c_int;
 }
 
 #[link(name = "util")]

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

@@ -608,6 +608,8 @@ extern {
     pub fn getentropy(buf: *mut ::c_void, buflen: ::size_t) -> ::c_int;
     pub fn pledge(promises: *const ::c_char,
                   paths: *mut *const ::c_char) -> ::c_int;
+    pub fn setresgid(rgid: ::gid_t, egid: ::gid_t, sgid: ::gid_t) -> ::c_int;
+    pub fn setresuid(ruid: ::uid_t, euid: ::uid_t, suid: ::uid_t) -> ::c_int;
 }
 
 cfg_if! {