[deb] Support for php7 and mariadb

- As discussed on https://sourceforge.net/p/vufind/mailman/message/35002108/
- These changes won't break the previous behaviour, they will just introduce new alternative dependencies based on virtual packages that can either provide php5 or php7 / mysql or mariadb
- Note that explicit apache2 php module dependency is needed because currently the virtual package php5 depends on libapache2-mod-php5 but the virtual package php uses php7-fpm by default, so by making the libapache2-mod-php package need explicit we won't run into cases where the user installed php7-fpm or the virtual package php choose php7-fpm instead of the apache's one.
- Note: Installing vufind without libapache2-mod-php or libapache2-mod-php5 fails with an apache errors as it doesn't recognize vufind.conf, which uses php keywords.
- Please also note that the first package in the precedence order [libapache2-mod-php5] is already a dependency from php5, so this won't break previous functionality.