Introduce fish_mutex_t wrapping std::mutex

Add a fish-specific wrapper around std::mutex that records whether it is
locked in a bool. This is to make ASSERT_IS_LOCKED() simpler (it can just
check the boolean instead of relying on try_lock) which will make Coverity
Scan happier.

Some details: Coverity Scan was complaining about an apparent double-unlock
because it's unaware of the semantics of try_lock(). Specifically fish
asserts that a lock is locked by asserting that try_lock fails; if it
succeeds fish prints an error and then unlocks the lock (so as not to leave
it locked). This unlock is of course correct, but it confused Coverity Scan.

src/env.cpp

@@ -162,7 +162,7 @@ class variable_entry_t {
     wcstring value; /**< Value of the variable */
 };
 
-static std::mutex env_lock;
+static fish_mutex_t env_lock;
 
 static bool local_scope_exports(const env_node_t *n);