Make SO_NOSIGPIPE best-effort in select backend

On develop, 5 of 6 SO_NOSIGPIPE call sites in the select backend
ignored setsockopt failures (best-effort); only the accept path was
strict, which was itself inconsistent with the rest of the backend.
The refactor propagated the strict behavior into the shared
set_fd_options helper, making all socket-creation paths fail on
SO_NOSIGPIPE errors — a behavior change from develop.

Switch both the accept_policy and set_fd_options to best-effort to
match develop's predominant behavior. Write paths still use
MSG_NOSIGNAL where available, so SO_NOSIGPIPE is a safety net, not
a hard requirement.

Author: mvandeberg

include/boost/corosio/native/detail/select/select_traits.hpp

@@ -140,16 +140,12 @@ struct select_traits
             }
 
 #ifdef SO_NOSIGPIPE
+            // Best-effort: SO_NOSIGPIPE is a safety net; write paths
+            // also use MSG_NOSIGNAL where available. Failure here
+            // should not prevent the accepted connection from being used.
             int one = 1;
-            if (::setsockopt(
-                    new_fd, SOL_SOCKET, SO_NOSIGPIPE,
-                    &one, sizeof(one)) == -1)
-            {
-                int err = errno;
-                ::close(new_fd);
-                errno = err;
-                return -1;
-            }
+            (void)::setsockopt(
+                new_fd, SOL_SOCKET, SO_NOSIGPIPE, &one, sizeof(one));
 #endif
 
             return new_fd;
@@ -178,15 +174,14 @@ struct select_traits
             return make_err(EINVAL);
 
 #ifdef SO_NOSIGPIPE
-        // SO_NOSIGPIPE is the primary defense against SIGPIPE on
-        // platforms that don't support MSG_NOSIGNAL. A silent failure
-        // here would leave the fd vulnerable to crashing the process
-        // on a closed-peer write, so propagate the error.
+        // Best-effort: SO_NOSIGPIPE is a safety net; write paths
+        // also use MSG_NOSIGNAL where available. Match develop's
+        // predominant behavior of ignoring failures here rather
+        // than failing socket creation.
         {
             int one = 1;
-            if (::setsockopt(
-                    fd, SOL_SOCKET, SO_NOSIGPIPE, &one, sizeof(one)) != 0)
-                return make_err(errno);
+            (void)::setsockopt(
+                fd, SOL_SOCKET, SO_NOSIGPIPE, &one, sizeof(one));
         }
 #endif