(As promised in #2496 . It may seem like a lot, but it is mainly some fairly automatic changes)

• Automatic prefix when using the syntax _tableName (with backticks)
  • This allows writing:

SELECT * FROM `_entry`;

instead of:

SELECT * FROM `' . $this->prefix . 'entry`; 

As well as writing:

return $this->pdo->lastInsertId('`_tag_id_seq`');

instead of:

return $this->bd->lastInsertId('"' . $this->prefix . 'tag_id_seq"');

• Uniformity: MySQL is now using PDO::ATTR_EMULATE_PREPARES = false just like SQLite and PostgreSQL, with consequences such as only one statement per query. See e.g. https://phpdelusions.net/pdo#emulation
• Use PDO methods exec(), query(), prepare() + execute() in a more efficient way
• Remove auto-update SQL code for versions older than FreshRSS 1.5 (3 years old)
• The name of the default category is set in PHP instead of in the DB (simplies SQL and allows changing the name according to the FreshRSS language)
• Rename ->bd to ->pdo (less of a frenshism, and more informative)
• Fix some requests, which were not compatible with MySQL prepared statements