[ticket/11358] Changed the name of post parameter.

Replaced the parameter name 'defaultbylink' to a meaningful name
'set_default_on_all'. Parameter is used for making all users
default for selected group in acp_groups.php.

PHPBB3-11358
[ticket/11358] Fixed the code according to coding guidlines.

Added proper indentation and new line after break in switch
statement.

PHPBB3-11358