[ticket/12413] Move deprecated functions to functions_compatibility.php

*Currently 13 functions are marked as deprecated:*
**functions.php:**
- function set_var()
- function request_var()
- function set_config()
- function set_config_count()
- function tz_select()
- function add_log()

**functions_admin:**
- function cache_moderators()
- function update_foes()

**functions_compatibility:**
- function get_user_avatar()
- function phpbb_hash()
- function phpbb_check_hash()
- function phpbb_clean_path()

**functions_install:**
- function get_tables()

*8 of them are still used in the core:*
- add_log              Used in many places (~198 calls)
- get_tables           Used in install\install_convert::get_convert_settings and functions_install.php\connect_check_db
- get_user_avatar      Used in phpbb\user_loader:get_avatar
- phpbb_hash           Used in phpbb\db\migration\data\v30x\release_3_0_5_rc1
- request_var          Used quite everywhere (~997 calls)
- set_config           Used in many places (~129 calls)
- set_config_count     Used in a few places (~37 calls)
- set_var              Used by phpbb_http_login and phpbb\auth\provider\apache::autologin

https://tracker.phpbb.com/browse/PHPBB3-12199

PHPBB3-12413