postgres: improve how PGHOST is computed

[sandydoo]

• Use one of the IP addresses from listen_addresses if configured.
• Handles special listen addresses, like *, 0.0.0.0, and ::.
• Only set PGPORT if using IP addresses. Reverted on main. PGPORT is required by initdb and other tools.
• Expand the docs for listen_addresses using the official postgres docs as reference.

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[schonfinkel]

@sandydoo thank you for these changes, I had once resorted to overriding the PGHOST set by devenv, to follow whatever value I had put in my listen_address.