Allow embedded spaces without quoting in unix_socket_directories entries.

This fix removes an unnecessary incompatibility with the old behavior of
the unix_socket_directory parameter.  Since pathnames with embedded spaces
are fairly popular on some platforms, the incompatibility could be
significant in practice.  We'll still strip unquoted leading/trailing
spaces, however.

No docs update since the documentation already implied that it worked
like this.

Per bug #7514 from Murray Cumming.