thread: mask all signals on child threads

Mask all signals on any threads we create.  Since we don't use
signals for anything, this leaves the signal behavior to the
original parent thread or process linking in librados or
libceph.

src/common/Thread.h

@@ -56,7 +56,12 @@ class Thread {
   }
   int create() {
     _num_threads.inc();
-    int r = pthread_create( &thread_id, NULL, _entry_func, (void*)this );
+    // mask signals in child's thread
+    sigset_t newmask, oldmask;
+    sigfillset(&newmask);
+    pthread_sigmask(SIG_BLOCK, &newmask, &oldmask);
+    int r = pthread_create(&thread_id, NULL, _entry_func, (void*)this);
+    pthread_sigmask(SIG_SETMASK, &oldmask, 0);
     generic_dout(10) << "thread " << thread_id << " start" << dendl;
     return r;
   }