Changed permissions for domain sockets to allow non-root executors.

Previously, the default permissions for domain sockets allowed
r/w access only for the file's user, so an executor launched under
a non-privileged user could not open the agent's socket. This patch
adds r/w permissions for the group and other users to address
the access problem.

Review: https://reviews.apache.org/r/72478
diff --git a/src/common/domain_sockets.hpp b/src/common/domain_sockets.hpp
index 6d2b0ab..630ea93 100644
--- a/src/common/domain_sockets.hpp
+++ b/src/common/domain_sockets.hpp
@@ -33,7 +33,7 @@
 namespace common {
 
 constexpr size_t DOMAIN_SOCKET_MAX_PATH_LENGTH = 108;
-constexpr int DOMAIN_SOCKET_DEFAULT_MODE = 0600;
+constexpr int DOMAIN_SOCKET_DEFAULT_MODE = 0666;
 
 
 inline Try<process::network::unix::Socket> createDomainSocket(