"unix_socket_directories" should be GUC_LIST_INPUT?

Hi

Not that I've ever had to do this (or would want to do it on a production
system), but this error message seems incorrect:

    postgres=# ALTER SYSTEM SET unix_socket_directories  =
'/tmp/sock1','/tmp/sock2';
    ERROR:  SET unix_socket_directories takes only one argument

Trivial patch attached.

Regards

Ian Barwick

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