Since mysqli can no longer be built against libmysql-client, there is
no longer the need to distinguish.
While we're at it, we also drop the superfluous is_object() checks.
Closes GH-9652.
- set default for mysqli.allow_local_infile=0
- explicitly disable PDO::MYSQL_ATTR_LOCAL_INFILE in case of lack of driver options
- add getAttribute support for PDO::MYSQL_ATTR_LOCAL_INFILE
- update existing tests where needed
- add new tests [checking default value and setting on] the 'local infile' in ext/mysqli and ext/pdo_mysql
