Dynamic certificates re-introduce problem with incorrect session
reuse (AKA "virtual host confusion", CVE-2014-3616), since there are
no server certificates to generate session id context from.

To prevent this, session id context is now generated from ssl_certificate
directives as specified in the configuration.  This approach prevents
incorrect session reuse in most cases, while still allowing sharing
sessions across multiple machines with ssl_session_ticket_key set as
long as configurations are identical.

Passwords have to be copied to the configuration pool to be used
at runtime.  Also, to prevent blocking on stdin (with "daemon off;")
an empty password list is provided.

To make things simpler, password handling was modified to allow
an empty array (with 0 elements and elts set to NULL) as an equivalent
of an array with 1 empty password.

This makes it possible to reuse certificate loading at runtime,
as introduced in the following patches.

Additionally, this improves error logging, so nginx will now log
human-friendly messages "cannot load certificate" instead of only
referring to sometimes cryptic names of OpenSSL functions.

The "(SSL:)" snippet currently appears in logs when nginx code uses
ngx_ssl_error() to log an error, but OpenSSL's error queue is empty.
This can happen either because the error wasn't in fact from OpenSSL,
or because OpenSSL did not indicate the error in the error queue
for some reason.

In particular, currently "(SSL:)" can be seen in errors at least in
the following cases:

- When SSL_write() fails due to a syscall error,
  "[info] ... SSL_write() failed (SSL:) (32: Broken pipe)...".

- When loading a certificate with no data in it,
  "[emerg] PEM_read_bio_X509_AUX(...) failed (SSL:)".
  This can easily happen due to an additional empty line before
  the end line, so all lines of the certificate are interpreted
  as header lines.

- When trying to configure an unknown curve,
  "[emerg] SSL_CTX_set1_curves_list("foo") failed (SSL:)".

Likely there are other cases as well.

With this change, "(SSL:...)" will be only added to the error message
if there is something in the error queue.  This is expected to make
logs more readable in the above cases.  Additionally, with this change
it is now possible to use ngx_ssl_error() to log errors when some
of the possible errors are not from OpenSSL and not expected to have
anything in the error queue.

Checking multiple errors at once is a bad practice, as in general
it is not guaranteed that an object can be used after the error.
In this particular case, checking errors after multiple allocations
can result in excessive errors being logged when there is no memory
available.