Auto merge of #602 - Razican:unistd_sync, r=alexcrichton

Added the sync() function from unistd.h

This fixes #601.

src/unix/bsd/mod.rs

@@ -432,6 +432,8 @@ extern {
                link_name = "recvmsg$UNIX2003")]
     pub fn recvmsg(fd: ::c_int, msg: *mut ::msghdr, flags: ::c_int)
                    -> ::ssize_t;
+
+    pub fn sync();
 }
 
 cfg_if! {

src/unix/notbsd/linux/mod.rs

@@ -1001,6 +1001,7 @@ extern {
                     flags: ::c_int) -> ::c_int;
     pub fn recvmmsg(sockfd: ::c_int, msgvec: *mut mmsghdr, vlen: ::c_uint,
                     flags: ::c_int, timeout: *mut ::timespec) -> ::c_int;
+    pub fn sync();
 }
 
 cfg_if! {