make it possible to change MYSQLD_STARTUP_TIMEOUT on Debian systems

In some cases (think SST, large memory allocation) it takes longer than 30 seconds for the Database server to become available, in which case the init script will report failure. This is especially problematic on systems running systemd, since systemd will erroneously record the service as not started and might at some other time try to launch a second mysqld instance (stop/restart also won't work).

There is already an environment variable referenced when launching (MYSQLD_STARTUP_TIMEOUT) but there seems to be no way to set it. The Debian way would be to specify it in /etc/default/mysql and have something like this inside the init script:

[ -r /etc/default/$NAME ] && . /etc/default/$NAME

This was already fixed for Redhat based distributions in MDEV-5068