Fix for issue #461.

Various places in both documentation and default emails to admins/moderators
use he/him/his to refer to generic users of unknown gender. This commit
changes all generic uses of male pronouns to use singular they/them/their.
It also changes "his/her" to "their", but leaves "he or she" unchanged due
to less trivial conjugation issues.  NEWS, AUTHORS.md, and OChangeLog are
unchanged.

[change] Move task files to task/ subdirectories. Previously list_task_models/ and global_task_models/ directories were used.  Task files in older directories will be automatically copied to new places during upgrading process.

[-bug] Even if it is obsoleted, parameters with occurrence '1' or '1-n' in config file log warnings.

Sympa::List::restore_users() for owner/editor replaces users on database with content of dump file, instead of simply adding content of dump file.

Reorganizing user interface (1).

  - Now sync_include_admin() passes cache and updates admin_table directly.
  - It no longer reload owner/editor in list config.

Known bug: Big datasource consumes large memory.

Options for sympa.pl are also renamed to --dump_users and --restore_users.

[bug] If a list is closed once and then reopened, list members are registered as permanent (not included) members.  Also, subscription suspension option are cleared and so on.
Fxied by dump and restore all information of subscribers in database.

They dump or restore list users (members, owners or editors) in database from/to file.

[change] File for members was changed to `<list dir>/member.dump` which will replace:
  - `subscribers` (used by Sympa prior to 5.4a.1)
  - `subscribers.db.dump` (generated by dump command)
  - `subscribers.closed.dump` (backup for closed list)
[change] Additional files to dump owner or editor: `<list dir>/owner.dump`, `<list dir>/editor.dump`.
They will be used as initial (permanent) owners/editors, or as backup for closed list
[*change] owner and editor parameters in list config file are no longer sync with actual owners/editors.  They are used only at the time of installation of list.

Fixed by:
  - Canonicalizing value of attrs1 as DN, if it is search base or root of it.
  - Escaping it as attributevalue in DN otherwise.

[change] pictures_url & pictures_path are no longer may be per-robot parameters.  Because list subdirectories under PICTURESDIR are assumed.

A tentative fix for issue #11.
  - Override owner/editor in edit_list page by information on database.
  - Don't use memory cache when admin is updated by sync_include_admin().
Fundamental fix will be done after the next stable.

Instead of these datetime fields, update_epoch_admin & update_epoch_subscriber will be used to compare timestamps easily.