Replace errno dependency with calling Error::last_os_error

The documentation for the errno crate specify that last_os_error fetch
the underlying errno in the exact same way, so this should be
equivalent, but with fewer dependencies

Cargo.toml

@@ -15,7 +15,6 @@ travis-ci = { repository = "mullvad/pfctl-rs" }
 
 
 [dependencies]
-errno = "0.2"
 error-chain = "0.12.4"
 ioctl-sys = "0.6.0"
 libc = "0.2.29"

src/macros.rs

@@ -15,8 +15,10 @@ macro_rules! ioctl_guard {
     };
     ($func:expr, $already_active:expr) => {
         if unsafe { $func } == $crate::macros::IOCTL_ERROR {
-            let ::errno::Errno(error_code) = ::errno::errno();
-            let io_error = ::std::io::Error::from_raw_os_error(error_code);
+            let io_error = ::std::io::Error::last_os_error();
+            let error_code = io_error
+                .raw_os_error()
+                .expect("Errors created with last_os_error should have errno");
             let mut err = Err($crate::ErrorKind::IoctlError(io_error).into());
             if error_code == $already_active {
                 err = err.chain_err(|| $crate::ErrorKind::StateAlreadyActive);