Implement suid sanboxing for PAM like MariaDB server does

Since MariaDB 10.4 PAM authentication is not handled by the MariaDB server process itself, but by separate sandbox processes running using suid privilege raising.

This has two advantages:

• potential crashes inside one of the pam_... shared libraries only bring down the sandbox process and not the actual server (MDEV-15473)

• no permission changes of files like /etc/shadow (has to be readable when using pam_unix.so) are needed, and neither does the server process itself have to run as root (MDEV-7032)

It would be a good thing to have the same for the PAM implementation on the maxscale side, too.