Improve check of whether mysql is(/should be) running in default prepare script

We think the intention of the old version of this block is

"if mysql is running, dump the databases".

The check has been buggy for a long long time: it reads the contents
of my.cnf ... which nowadays just has some !includedir directives.
This lead to setting

SOCKET=""

and, it turns out, [ -S "" ] returns true.

The main intention of this part of the prepare script is to backup
vaguely normal machines running a simple mysql database, such as
small group webservers. We thus don't need to consider every
eventuality.

ROOT/etc/chem-zfs-backup-server/zfs-rsync.d/prepare

@@ -74,9 +74,18 @@ which rsync >/dev/null 2>&1 || apt-get install rsync
 debconf-get-selections  >/var/adm/backup/debconf
 
 # Dump MySQL, if the root user has access
-if [ -f /etc/mysql/my.cnf ] ; then
- SOCKET=`awk ' BEGIN {FLAG=0} (\$0 ~ /^\[/) {FLAG=0} (\$0 ~ /^\[mysqld\]/) {FLAG=1} (\$1=="socket" && FLAG==1) { print \$3 ; } ' </etc/mysql/my.cnf`
- [ -S \$SOCKET ] && which mysqldump >/dev/null 2>&1 && mysqldump  --defaults-file=/etc/mysql/debian.cnf --all-databases >/var/adm/backup/mysql
+set +e
+systemctl is-enabled mysql --quiet 2>/dev/null
+MYSQL_ENABLED=\$?
+set -e
+
+if [ \$MYSQL_ENABLED -eq 0 ] ; then
+  if [ -f /root/.my.cnf ] ; then
+    DEFAULTS_FILE=/root/.my.cnf
+  else
+    DEFAULTS_FILE=/etc/mysql/debian.cnf
+  fi
+  mysqldump  --defaults-file=\$DEFAULTS_FILE --all-databases >/var/adm/backup/mysql
 fi
 
 # Ignore exit status of this command