Macro journald::sd_try

macro_rules! sd_try {
    ($e:expr) => { ... };
}

An analogue of try!() for systemd FFI calls.

The parameter should be a call to a systemd FFI fn with an c_int return value. It is called, and if the return is negative then sd_try!() interprets it as an error code and returns IoError from the enclosing fn. Otherwise, the value of sd_try!() is the non-negative value returned by the FFI call.