Historically, developers had to use the `eval` attribute to set
a False value for boolean fields in XML data files:

    <field name="active" eval="False"/>

This is a source of errors for beginners, who tend to provide
a string value directly, as for char/text fields.
Unfortunately string values always evaluated to `True` booleans:

    <field name="active">False</field> <!-- active == true! -->

This patch adds detection of some well-known falsy strings
(case-insensitive): `0`, `false`, `off`, as similarly supported
for the `noupdate` and `forcecreate` attributes of XML data records.

Closes #13152

The API of `Registry` has been modified as follows:
 - `Registry(name)` returns the registry of the given database
 - `Registry.new(name)` forces the creation of a new `Registry` object
 - `Registry.delete(name)` discards the given registry
 - `Registry.delete_all()` discards all registries
 - `registry.setup_signaling()` initializes the signaling for `registry`
 - `registry.check_signaling()` returns an up-to-date `registry`
 - `registry.signal_registry_change()` notifies that `registry` has changed
 - `registry.signal_caches_change()` notifies potential cache invalidation

This is an old compatibility code. It is time to remove it as it is not used
sinces ages.

The purpose of this commit is to clean a bit the code related to email
routing in the mailgateway: message_route and message_route_verify.

message_route is now clearly separeted into two categories. First one
handles emails considered as answers to existing threads. Second one
handles emails that have to create new threads, either through alias or
default behavior of the mailgateway.

message_route_verify has been a bit facotrized to simplify the various
overrides that appeared lately in the codebase.

This commit only contains code cleaning. No functional change should occur
due to this commit.

mail.mail and mail.thread have methods to decode headers. This commit
move them in mail tools. It is indeed simplier to have all email tools
in the same file. Moreover they have been renamed to avoid confusion
with standard library methods: decode and decode_header already exist.

 * decode in mail_message is now decode_smtp_header
 * decode_header in mail_thread is now decode_message_header

A method to data from references is also moved into tools. A regex was
already defined in tools. Now the method using it is already put in tools.
Mailgateway will now simply call this method.