Add support for MySQL and PostgreSQL database backups in configuration

conf/borg2-backup-nc.conf.sample

@@ -118,6 +118,60 @@
 # DB related parameters
 # ---
 
+# DB_TYPE
+#
+# Which additional database(s) should be backed up (in addition to the Nextcloud DB)?
+#
+# The Nextcloud database is always backed up automatically via 'occ'.
+# Use this parameter to also back up other (non-Nextcloud) databases on this host.
+#
+# Possible values:
+#     - MySQL                  (or mysql)
+#     - PostgreSQL             (or postgres or psql)
+#     - MySQL,PostgreSQL       (or MySQL PostgreSQL - comma or space separated)
+#     - None
+#
+# When set to MySQL: all MySQL databases except the Nextcloud DB and system
+# databases (information_schema, performance_schema, mysql, sys) are backed up.
+#
+# Defaults to:
+#
+#     DB_TYPE="None"
+#
+#DB_TYPE="None"
+
+
+# MYSQL_CREDENTIALS
+#
+# Giving password on command line is insecure and since mysql 5.5
+# you will get a warning doing so.
+#
+# Reading username/password from file is also possible, using MySQL/MariaDB
+# commandline parameter '--defaults-file'.
+#
+# Since Version 5.6, that method is considered as insecure.
+# To avoid giving the password on command line, we use an
+# encrypted option file.
+#
+#    Create (encrypted) option file:
+#    $ mysql_config_editor set --login-path=local --socket=/var/run/mysqld/mysqld.sock  --user=backup --password
+#    $ Password:
+#
+#    Use of option file:
+#    $ mysql --login-path=local ...
+#
+# Example
+#    MYSQL_CREDENTIALS="--login-path=local"
+#    MYSQL_CREDENTIALS="--defaults-file=/etc/mysql/debian.cnf" (Debian default)
+#    MYSQL_CREDENTIALS="--defaults-file=/usr/local/mysql/sys-maint.cnf"
+#
+# Defaults to:
+#
+#     MYSQL_CREDENTIALS="-u root -S /run/mysqld/mysqld.sock"
+#
+#MYSQL_CREDENTIALS="-u root -S /run/mysqld/mysqld.sock"
+
+
 # MYSQL_PERFORMANCE_SETTINGS
 #
 # To increase performance, some global variables are changed before writing the database dump.
@@ -140,13 +194,24 @@
 
 # MYSQL_MAX_ALLOWED_PACKET
 #
-# Set/Change this value only for dumping the NC database
+# Maximum packet size for mysqldump (applies to NC DB and additional MySQL databases)
 #
 # Defaults to:
 #
-#     #MYSQL_MAX_ALLOWED_PACKET=1G
+#     MYSQL_MAX_ALLOWED_PACKET="512M"
 #
-#MYSQL_MAX_ALLOWED_PACKET=1G
+#MYSQL_MAX_ALLOWED_PACKET="512M"
+
+
+# PSQL_SYSTEM_USER
+#
+# User who owns PostgreSQL services
+#
+# Defaults to:
+#
+#     PSQL_SYSTEM_USER="postgres"
+#
+#PSQL_SYSTEM_USER="postgres"
 
 
 # ---