HEADS-UP: struct pam_message in conversation function is now const

HEADS-UP: struct pam_message in conversation function is now const

[Andy Fiddaman]

(I'm sending this out on Dominik's behalf):

"15657 struct pam_message in conversation function should be const" has
recently been integrated and it updates the definition of the pam_message
parameter to a conversation function to be const. This brings it in line
with what was already in at least one man page
(https://illumos.org/man/3PAM/pam_start),
and with other systems such as FreeBSD and Linux.

This should improve compatibility with 3rd party software which already
generally expects this. However, some software has conditionals based on
illumos/Solaris to select a non-const variant on those platforms.
(A good example is
https://github.com/openssh/openssh-portable/blob/V_9_8/auth-pam.c#L70-L75)
Doing so is usually just a compiler warning, but can become an error if
software is:

- built with clang >= 16
- built with gcc >= 14
- built with `-Werror`

We are working on patching 3rd party software upstream. If you hit a
problem, please reach out, but you can also define `_PAM_LEGACY_NONCONST`
as a last resort to get back the legacy non-const interface.

Regards,

Dominik