Supporting TCP_SYNCNT in libpq

This patch introduces support for a `tcp_syn_count` parameter in
libpq, allowing control over the number of SYN retransmissions when
initiating a connection.

The primary goal is to prevent the walreceiver from getting stuck
resending SYNs for an extended period, up to
`net.ipv4.tcp_syn_retries` (127 seconds by default), in the event of
network disruptions.

A specific scenario where this can occur is during a failover in Kubernetes:

* The primary node fails, a standby is promoted, and other standbys
attempt to reconnect to the service representing the new primary.
* The `primary_conninfo` of the standby points to a service, usually
managed via iptables rules.
* If the walreceiver's initial SYN is dropped due to outdated rules,
the connection may remain stranded until the system timeout is
reached.
* As a result, a second standby may reattach after a couple of
minutes. In the case of synchronous replication, this can block the
writes from the application.

In this scenario, `tcp_user_timeout` could close a connection retrying
the SYNs (even though it doesn't seem to do it from the documentation,
it works) the parameter will affect the entire connection.
`connect_timeout`, doesn't work with `PQconnectPoll`, so it won't
prevent the walreceiver from timing out.

Thank you,
Francesco