Remove reference to struct execve_args from struct imgact, which

Remove reference to struct execve_args from struct imgact, which
describes an image activation instance.  Instead, make use of the
existing fname structure entry, and introduce two new entries,
userspace_argv, and userspace_envv.  With the addition of
mac_execve(), this divorces the image structure from the specifics
of the execve() system call, removes a redundant pointer, etc.
No semantic change from current behavior, but it means that the
structure doesn't depend on syscalls.master-generated includes.

There seems to be some redundant initialization of imgact entries,
which I have maintained, but which could probably use some cleaning
up at some point.

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, Network Associates Laboratories