[PATCH] Add `verify-system` sslmode to use system CA pool for server cert

With Letsencrypt now protecting web servers left and right, and it makes
sense to me to just re-use the cert that the server may already have
installed.

I've tested this on debian with the client compiled from the master branch,
against a 13.3 server.

This is my first patch to postgresql, so I apologize for any process
errors. I tried to follow
https://wiki.postgresql.org/wiki/Submitting_a_Patch

Hope this list takes attachments.

--
typedef struct me_s {
char name[] = { "Thomas Habets" };
char email[] = { "thomas(at)habets(dot)se <thomas(at)habets(dot)pp(dot)se>" };
char kernel[] = { "Linux" };
char *pgpKey[] = { "http://www.habets.pp.se/pubkey.txt" };
char pgp[] = { "9907 8698 8A24 F52F 1C2E 87F6 39A4 9EEA 460A 0169" };
char coolcmd[] = { "echo '. ./_&. ./_'>_;. ./_" };
} me_t;