Duplicate ACEs are merged but should be rejected

In the authz semantics up to 1.9, a duplicate access entry for the same rule would replace a previous such entry:

[/]
user = rw
user = r

This was valid, and the second entry replaced the first, giving user read-only access.

In 1.10+, these entries are merged, giving user rear/write access. This is clearly bad because it can silently change the meaning of access rules.

Proposal: duplicate authz rules should be rejected, i.e., the example above should become an error. Whilst this will break some existing pre-1.10 authz files, it will not silently change their meaning. Besides, duplicate entries are most likely either an error or the result of duplicate rules, which are also forbidden in 1.10+.

The attached patch implements this proposal. Examples:

$ cat authz.conf 
[/]
user = rw
user = r
$ svnauthz validate authz.conf 
svnauthz: E220003: Error while parsing authz file: 'authz.conf':
svnauthz: E220003: Duplicate access entry 'user' in rule [/]

$ cat authz.conf 
[/]
$authenticated = rw
~$anonymous = r
$ svnauthz validate authz.conf 
svnauthz: E220003: Error while parsing authz file: 'authz.conf':
svnauthz: E220003: Duplicate access entry '~$anonymous' (matches '$authenticated') in rule [/]

$ cat authz.conf 
[aliases]
resu = user

[/]
~&resu = rw
~user = r
$ svnauthz validate authz.conf 
svnauthz: E220003: Error while parsing authz file: 'authz.conf':
svnauthz: E220003: Duplicate access entry '~&resu' (matches '~user') in rule [/]