kernel-install: allocate "Context" object only in verb_xyz() functions, not already...

kernel-install: allocate "Context" object only in verb_xyz() functions, not already in run()

We soon want to add a Varlink interface to this, but that means that the
various paramaters for the Context object will be sourced from a Varlink
message not from the command line. Hence split apart the parsing logic
so that we alway parse the command line into arg_xyz first, and then,
inside the verb_abc() calls copy the data from there into the Context
object.

This matches a similar pattern in bootctl.