environment: Properly handle SIGINT.

environment: Properly handle SIGINT.

Switching to execlp means that the process spawned in a container is PID
1, which obsoleted one of the 'guix environment --container' tests
because the init process can't be killed in the usual manner.

* guix/scripts/environment.scm (launch-environment/fork): New procedure.
(launch-environment): Switch from system* to execlp.  Add handler for
SIGINT.
(guix-environment): Use launch-environment/fork.
* tests/guix-environment-container.sh: Replace abnormal exit test with
one that works now that the spawned process is PID 1.