Add PQencryptPasswordConn function to libpq, use it in psql and createuser.

The new function supports creating SCRAM verifiers, in addition to md5
hashes. The algorithm is chosen based on password_encryption, by default.

This fixes the issue reported by Jeff Janes, that there was previously
no way to create a SCRAM verifier with "\password".

Michael Paquier and me

Discussion: https://www.postgresql.org/message-id/CAMkU%3D1wfBgFPbfAMYZQE78p%3DVhZX7nN86aWkp0QcCp%3D%2BKxZ%3Dbg%40mail.gmail.com

src/interfaces/libpq/libpq-fe.h

@@ -597,6 +597,7 @@ extern int	PQenv2encoding(void);
 /* === in fe-auth.c === */
 
 extern char *PQencryptPassword(const char *passwd, const char *user);
+extern char *PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user, const char *algorithm);
 
 /* === in encnames.c === */