Allow merging of similarly named text domains

[sambauers]

This is mostly to allow the 'default' textdomain to be loaded from two different .mo files.

The problem is described in detail at ​http://trac.bbpress.org/ticket/765

We need this change in WordPress because the WordPress l10n.php functions take precedence over the file in bbPress.

[ryan]

(In [8556]) Allow merging of similarly named text domains. Props sambauers. fixes #7376 for trunk

[ryan]

(In [8557]) Allow merging of similarly named text domains. Props sambauers. fixes #7376 for 2.6

[hakre]

Related: #10286, #10527