mariadb: update auth_pam handling

mariadb: update auth_pam handling

With INSTALL_PAMDIR undefined some items necessary for the auth_pam
module aren't built. This adds the define so that configuration and
shared object become available.

This commit also tightens up the installation of the SUID tool. The
directory it is copied into gets created on the build host already with
u=rwx,g=rx,o=, so it cannot be accessed on target, except by root. The
post-install script then changes group ownership of the directory to the
"mariadb" group only if the directory is really a directory and owned by
"root:root".

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>