libpq sslpassword parameter and callback function

This patch provides for an sslpassword parameter for libpq, and a hook
that a client can fill in for a callback function to set the password.

This provides similar facilities to those already available in the JDBC
driver.

There is also a function to fetch the sslpassword from the connection
parameters, in the same way that other settings can be fetched.

This is mostly the excellent work of my colleague Craig Ringer, with a
few embellishments from me.

Here are his notes:

    Allow libpq to non-interactively decrypt client certificates that
are stored
    encrypted by adding a new "sslpassword" connection option.
   
    The sslpassword option offers a middle ground between a cleartext
key and
    setting up advanced key mangement via openssl engines, PKCS#11, USB
crypto
    offload and key escrow, etc.
   
    Previously use of encrypted client certificate keys only worked if
the user
    could enter the key's password interactively on stdin, in response
to openssl's
    default prompt callback:
   
        Enter PEM passhprase:
   
    That's infesible in many situations, especially things like use from
    postgres_fdw.
   
    This change also allows admins to prevent libpq from ever prompting
for a
    password by calling:
   
        PQsetSSLKeyPassHook(PQdefaultSSLKeyPassHook);
   
    which is useful since OpenSSL likes to open /dev/tty to prompt for a
password,
    so even closing stdin won't stop it blocking if there's no user
input available.
    Applications may also override or extend SSL password fetching with
their own
    callback.
   
    There is deliberately no environment variable equivalent for the
sslpassword
    option.

cheers

andrew

--
Andrew Dunstan https://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services