On Linux, both Thunderbird and Firefox use the same directory to contain
native messaging host modules. On this platform, we have to merge both
directories with native hosts into one.
The patch introduces a separate helper module to manage native host
directory generation. Now program modules (firefox, thunderbird) declare
native hosts to initialize; while the new helper module determines
*where* and *how* to merge them on disc.
Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>
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