[ticket/11358] Success message even without selecting a user.

In group membership management, if you perform the action
'Make group default for member' without selecting any user,
a confirm box will be displayed and will show a success
message after confirming. Added a new condition to fix this
issue in acp_groups.php. It will check weather any users are
selected before performing above action.

PHPBB3-11358
[ticket/11358] Removed redundant code and referred proper variable.

Error was thrown when no users are selected before executing the code
chunk which manually adds all the user to make the group default.
Removed this code and referred $mark_ary instead of $name_ary which
will not have selected user ids.

PHPBB3-11358
[ticket/11358] Enabled link making all users default for a group.

Detect whether link is clicked or form is posted for making users
default for a group. If form is posted and no users are selected,
validation happens and displays an error message. When the link is
clicked, all the users will be default users for that group.

PHPBB3-11358
[ticket/11358] Changed the action parameter value to represent the link.

Changed the action parameter value to recognize whether 'Make default
group for every member' is clicked. If so execute a seperate code block
under switch statement for $action and add all the users as default
for the group.

PHPBB3-11358
[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
Merge branch 'ticket/11358' into develop-olympus

* ticket/11358:
  [ticket/11358] Changed the name of post parameter.
  [ticket/11358] Changed the action parameter value to represent the link.
  [ticket/11358] Enabled link making all users default for a group.
  [ticket/11358] Removed redundant code and referred proper variable.
  [ticket/11358] Success message even without selecting a user.