Refactor sysvmq::unlink_* and check for error

Use `sysvmq::id_from_key(key)` to receive the id of a queue identified
with `key`. Here an error check is added as well. Adapt
`Sysv::Unlink::run()` to comply with that change.

src/lib.rs

@@ -69,10 +69,13 @@ pub fn unlink_id(id: i32) -> Result<(), SysvMqError> {
     }
 }
 
-pub fn unlink_key(key: i32) -> Result<(), SysvMqError> {
+pub fn id_from_key(key: i32) -> Result<i32, SysvMqError> {
     let id = unsafe { msgget(key, 0) };
 
-    unlink_id(id)
+    match id {
+        -1 => Err(SysvMqError::ErrnoError(Errno::from_i32(errno()).desc())),
+        id => Ok(id),
+    }
 }
 
 pub struct SysvMq<T> {