authname field in authmap table must be unique, so should be namespaced

Discussed this with snufkin the other day...

Recently came across a site migrating from using LDAP to simplesaml. The site still had entries in the authmap table from the LDAP integration e.g.

+-----+-----+----------+-----------+
| aid | uid | authname | module    |
+-----+-----+----------+-----------+
|   1 | 123 | username | ldap_user |
+-----+-----+----------+-----------+

This caused a problem when simplesamlphp_auth tried to add its own entries to the authmap table, as it did so with the same value for authname (based on the user name), and there's a UNIQUE constraint on that field in the db schema:

> DESCRIBE authmap;
+----------+------------------+------+-----+---------+----------------+
| Field    | Type             | Null | Key | Default | Extra          |
+----------+------------------+------+-----+---------+----------------+
| aid      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| uid      | int(11)          | NO   |     | 0       |                |
| authname | varchar(128)     | NO   | UNI |         |                |
| module   | varchar(128)     | NO   |     |         |                |
+----------+------------------+------+-----+---------+----------------+

It's perhaps not particularly well documented, but it looks like in order to support the ability to have more than authmap entry for a given user (which the docs for user_set_authmaps suggest should be possible), the authname should be namespaced by each module.

See #920908: {authmap}.authname should not be unique. and #1118422: Document unique key on authname in {authmap}.

So perhaps something like:

+-----+-----+-----------------------------+--------------------+
| aid | uid | authname                    | module             |
+-----+-----+-----------------------------+--------------------+
|   1 | 123 | username@ldap_user          | ldap_user          |
|   2 | 123 | username@simplesamlphp_auth | simplesamlphp_auth |
+-----+-----+-----------------------------+--------------------+

I'll look at whether it's appropriate to file a companion issue for LDAP too.