Thanks for your advice.
After some further investigation, I discovered:
- Yes, I do need to specify "--dbtype=mariadb" in my options to the install script, when I'm using MariaDB.
- The install script doesn't seem to have an option to allow me to specify the "dblibrary", but 'native' does seem to end up as the setting which gets written into the config.php file after installation anyway.
- On Debian, at least, I do need to install the php5-mysqlnd package to enable the PHP database connection functionality, and the reason that it wasn't working for me at first is because I had initially forgotten the need to restart Apache after installing the library, oops!
Moodle seems to be working OK for me now, thanks!