libpq: Allow IP address SANs in server certificates

libpq: Allow IP address SANs in server certificates

The current implementation supports exactly one IP address in a server
certificate's Common Name, which is brittle (the strings must match
exactly).  This patch adds support for IPv4 and IPv6 addresses in a
server's Subject Alternative Names.

Per discussion on-list:

- If the client's expected host is an IP address, we allow fallback to
  the Subject Common Name if an iPAddress SAN is not present, even if
  a dNSName is present.  This matches the behavior of NSS, in
  violation of the relevant RFCs.

- We also, counter-intuitively, match IP addresses embedded in dNSName
  SANs.  From inspection this appears to have been the behavior since
  the SAN matching feature was introduced in acd08d76.

- Unlike NSS, we don't map IPv4 to IPv6 addresses, or vice-versa.

Author: Jacob Champion <pchampion@vmware.com>
Co-authored-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Co-authored-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/flat/9f5f20974cd3a4091a788cf7f00ab663d5fcdffe.camel@vmware.com