openssl_pkey_new() fetches various options from the config file --
most of these are optional, and not specifying them is not an error
condition from the perspective of the user. Unfortunately, the
CONF_get_string() API pushes an error when accessing a key that
doesn't exist (_CONF_get_string does not, but that is presumably a
private API). This commit adds a helper php_openssl_conf_get_string()
that automatically clears the error in this case. I've found that
OpenSSL occasionally does the same thing internally:
https://github.com/openssl/openssl/blob/22040fb790c854cefb04bed98ed38ea6357daf83/apps/req.c#L515-L517

Closes GH-6699.

If RSA key generation fails, actually report that failure.

We need to first clean the symtable and then check whether a cache
slot is available for it. Otherwise, it may happen that a destructor
runs while cleaning the table and uses up all the remaining slots
in the cache.

This is particularly insidious because once we overflow the cache,
the first pointer we modify is symtable_cache_ptr, making it hard
to understand what happened after the fact.

Fixes oss-fuzz #30815.

The PAM service requires the terminating null to be part
of the communication.

Tested with MariaDB-10.4(pam) and Percona Server 5.7.32(auth_pam_compat).

Also changed MySQL Enterprise test to the server side plugin, authentication_pam
as opposed to the client plugin mysql_clear_password.

Add additional check for pamtest user and pam service file as
all are required for the test.

More importantly, test result should actually succeed.

Thanks Geoff Montee for bug report.

Closes GH-78680.

We need to always destroy current, not just when iter.data is not
set.

Take this opportunity to clean up the iterator destructor code a
bit, to remove redundant checks and incorrect comments.

This breaks the build for PHP 8 by default.

Version numbers are not supposed to be localized, so we must not apply
locale dependent parsing with `atof()`.

Using `php_version_compare()` might even be better.

Closes GH-6668.