tx11ssh.c: replace 'char * socket_file' with 'char * socket_file_name'

The combination of

apparently made socket_file_size expand as G.G.socket_file in some
systems (one being gcc version 4.8.2 (GCC) Target: x86_64-pc-cygwin).

Changing socket_file to socket_file_name and the above define as
fixes this ambiquity.