[ticket/12199] 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

PHPBB3-12199
Merge pull request #2345 from Nicofuma/ticket/12199

[ticket/12199] Move deprecated functions to functions_compatibility.php

* Nicofuma/ticket/12199:
  [ticket/12199] Move deprecated functions to functions_compatibility.php
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12199] Move deprecated functions to functions_compatibility.php