Support for environment passing.

Now it is possible to specify additional environment strings for an
application in the manifest by adding an 'environment' key-value
mapping to it.
Declare environ explicitly as in environ(7).
Ensure app endpoint uniqueness with getpid(2).

If the runtime crashes, then it will not restart any of the apps if
restarted itself, because stale app unix sockets won't allow to create
the new ones.
Changelog bump.
Reset the actor's channel on close().
Merge branch 'master' of github.com:cocaine/cocaine-core
Named threads support on Linux using prctl(2).
Loggers now have access to the runtime config.
Move the node's uuid to the config.