init_tcp_socket: close socket fd on errors

The current code may leak the fd if bind fails, for example.  This is
a benign bug however, since all paraslash applications that call
init_tcp_socket() terminate on errors anyway.

But it is no good idea to rely on this, so close the fd on errors.

This patch also gets rid of the do_bind() one-liner which was only
called from init_tcp_socket().