(2520) Implements manual directory environments

Those are created with an environment_timeout value of "manual".
Those environments are cached indefinitely unless their directories
are manually touched (ie its ctime must change).

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>
(2520) support TTL unmunging for displaying the original value
(2520) add a face to list environments and flush 'manual' ones

This face allows to:
* list defined environments (including details with --details)
* flush one,several or all environements (this touches the environment
directory).

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>