implement method to terminate a job created by vifm.startjob

[dacyberduck]

once a job is created, there is no suitable way to stop it on demand. It will be useful to have a method that will return the pid of the child process so that it can be terminated on demand.

[xaizek]

There is already "terminate job" function in C which just isn't exposed to Lua. It has advantage or working on Windows the same way. Can add a method to get PID as well for some other purpose (:terminate() will do kill -9).