[3.0] Reserve the names on the reserved names list

[albertlast]

Description

A fresh 3.0 forum reserves nothing. Admin, Webmaster, Guest and root are
all free to register, and so is anything an admin adds afterwards — until they
open Registration → Reserved Names and press Save, which is the only thing that
ever writes the setting properly.

The stored value on a fresh install:

$ mysql -e "SELECT HEX(value) FROM smf_settings WHERE variable='reserveNames'"
41646D696E5C6E5765626D61737465725C6E47756573745C6E726F6F74
                ^^^^                ^^^^         ^^^^

5C6E is backslash-n, two characters, not a newline. checkReservedName() does
explode("\n", …) and gets one element back: the whole list as a single name
that nobody would ever type.

Where it comes from. The default is a language string that spells its
separators that way:

$txt['default_reserved_names'] = 'Admin\nWebmaster\nGuest\nroot';

2.1 fed that straight into an SQL literal — plain '…' for MySQL, E'…' for
PostgreSQL — and both engines turned the escapes into newlines on the way in.
3.0 defines its schema in PHP and inserts the value as a query parameter, so
nothing unescapes it. The strtr() in Table.php that survives from 2.1 only
turns \n back into \n; it was compensating for the escaping that the SQL
literal then undid, and there is no SQL literal any more.

Why nobody noticed. Admin/Registration.php has carried
str_replace('\n', "\n", …) since 2.1, so the textarea splits the list onto
separate lines and it looks exactly right. It has just never been in force.

What changes

Seeds real newlines, so a new install is correct.

Reads either form in checkReservedName(), so a forum that is already installed
starts enforcing its list without the admin having to go and re-save a page they
have no reason to think is broken. That is also why this needs no migration.

Testing

On a forum installed before the change, ?action=signup;sa=usernamecheck;xml:
root, Webmaster and Guest come back valid="0"; r00t and zzznewuser
come back valid="1". Submitting a registration as Webmaster is refused with
the name in the error box. A name not on the list registers as before.

(Read together with #9483, which is what makes the refusal a valid="0" rather
than an error page. Each stands alone.)

Issues References (Fixes|Related|Closes)

Related to #7933