Disable prompting for passphrase while (re)loading SSL config files.

OpenSSL's default behavior when loading a passphrase-protected key file
is to open /dev/tty and demand the password from there.  It was kinda
sorta okay to allow that to happen at server start, but really that was
never workable in standard daemon environments.  And it was a complete
fail on Windows, where the same thing would happen at every backend launch.
Yesterday's commit de41869b put the final nail in the coffin by causing
that to happen at every SIGHUP; even if you've still got a terminal acting
as the server's TTY, having the postmaster freeze until you enter the
passphrase again isn't acceptable.

Hence, override the default behavior with a callback that returns an empty
string, ensuring failure.  Change the documentation to say that you can't
have a passphrase-protected server key, period.

If we can think of a production-grade way of collecting a passphrase from
somewhere, we might do that once at server startup and use this callback
to feed it to OpenSSL, but it's far from clear that anyone cares enough
to invest that much work in the feature.  The lack of complaints about
the existing fractionally-baked behavior suggests nobody's using it anyway.

Discussion: https://postgr.es/m/29982.1483412575@sss.pgh.pa.us