LibCore: Prefer strlcpy over strncpy, fix overflow

A malicious caller can create a SocketAddress for a local unix socket with an
over-long name that does not fit into struct sock_addr_un.
- Socket::connet: This caused the 'sun_path' field to
  overflow, probably overwriting the return pointer of the call frame, and thus
  crashing the process (in the best case).
- SocketAddress::to_sockaddr_un: This triggered a RELEASE_ASSERT, and thus
  crashing the process.

Both have been fixed to return a nice error code instead of crashing.

Libraries/LibCore/LocalServer.cpp

@@ -121,7 +121,12 @@ bool LocalServer::listen(const String& address)
 #endif
 
     auto socket_address = SocketAddress::local(address);
-    auto un = socket_address.to_sockaddr_un();
+    auto un_optional = socket_address.to_sockaddr_un();
+    if (!un_optional.has_value()) {
+        perror("bind");
+        return false;
+    }
+    auto un = un_optional.value();
     rc = ::bind(m_fd, (const sockaddr*)&un, sizeof(un));
     if (rc < 0) {
         perror("bind");