smtpsrv: Always sanitize HELO/EHLO domain

Today we use the HELO/EHLO domain in a variety of places, which are
informational, but some have specific syntax (like the Received header).
Using the un-sanitized values can lead to some confusing and/or invalid
values. This is not very risky today, but it could be problematic in the
future if these strings are used in other contexts.

To prevent those problems, this patch sanitizes the HELO/EHLO domain
when receiving it.

Hooks used to receive the sanitized and raw variants; now, both are
sanitized (the raw one is kept for backwards compatibility).

If the sanitization leaves the string empty, we use the hard-coded
no-valid-characters.invalid as a placeholder, to make this situation
easier to identify when troubleshooting.

docs/hooks.md

@@ -42,9 +42,10 @@ The environment will contain the following variables:
  - `$PWD`: The working directory, which will be the config directory.
  - `$REMOTE_ADDR`: IP address of the remote side of the connection.
  - `$EHLO_DOMAIN`: EHLO/HELO domain, as given by the client; sanitized for
-   safety.
- - `$EHLO_DOMAIN_RAW`: Same as `$EHLO_DOMAIN`, but not sanitized; be careful as
-   it can contain problematic characters.
+   safety (only letters, digits, and `-.[]:` are kept). If there are no valid
+   characters, it is `no-valid-characters.invalid`.
+ - `$EHLO_DOMAIN_RAW`: Same as `$EHLO_DOMAIN`. Kept for backwards
+   compatibility; it used to be the unsanitized value.
  - `$MAIL_FROM`: MAIL FROM address.
  - `$RCPT_TO`: RCPT TO addresses, space separated.
  - `$AUTH_AS`: Authenticated user; empty if the connection has not