Make server_audit_excl_users have effect on connection logging

Simple test

INSTALL PLUGIN server_audit SONAME 'server_audit';
set global server_audit_logging=1;
set global server_audit_events=connect;
set global server_audit_excl_users=root;

Login/out

Connections are still logged:
20161023 11:52:40,thinkpad,root,localhost,8,13,QUERY,,'set global server_audit_logging=1',0
20161023 11:53:05,thinkpad,root,localhost,8,0,DISCONNECT,,,0
20161023 11:53:09,thinkpad,root,localhost,9,0,CONNECT,,,0
20161023 11:53:10,thinkpad,root,localhost,9,0,DISCONNECT,,,0
20161023 11:53:46,thinkpad,root,localhost,10,0,CONNECT,,,0
20161023 11:54:12,thinkpad,root,localhost,10,0,DISCONNECT,,,0
20161023 11:54:15,thinkpad,root,localhost,11,0,CONNECT,,,0
20161023 11:54:17,thinkpad,root,localhost,11,0,DISCONNECT,,,0