Change SigAction::flags to use from_bits_truncated

On Linux, if the signal trampoline code is in the C library, sigaction
sets the SA_RESTORER flag (0x04000000) in the sa_flags field of old
sigaction (see sigreturn(2)).

This is not intended for application use and is missing from SaFlags,
therefore from_bits fails and unwrapping panics the user program.

This fix just drops the bits that are not defined in SaFlags.

src/sys/signal.rs

@@ -389,7 +389,7 @@ impl SigAction {
     }
 
     pub fn flags(&self) -> SaFlags {
-        SaFlags::from_bits(self.sigaction.sa_flags).unwrap()
+        SaFlags::from_bits_truncate(self.sigaction.sa_flags)
     }
 
     pub fn mask(&self) -> SigSet {