MDEV-11159 Server proxy protocol support

accept proxy protocol header from client connections.
The new server variable 'proxy_protocol_networks' contains list
of networks from which proxy header is accepted.

libmysqld/CMakeLists.txt

@@ -117,6 +117,7 @@ SET(SQL_EMBEDDED_SOURCES emb_qcache.cc libmysqld.c lib_sql.cc
            ../sql/ha_sequence.cc ../sql/ha_sequence.h
            ../sql/temporary_tables.cc
            ../sql/session_tracker.cc
+           ../sql/proxy_protocol.cc
            ${GEN_SOURCES}
            ${MYSYS_LIBWRAP_SOURCE}
 )