Construct a struct image_args with the help of new exec_args_*() helper
functions and call kern_execve().
The previous code mapped a page in userspace, copied arguments out
to it one at a time, and then constructed a struct execve_args all so
that sys_execve() can call exec_copyin_args() to copy the data back in
to a struct image_args.
Opencode the part of pre_execve()/post_execve() that releases a
reference to the initial vmspace. We don't need to stop threads like
they do.
Reviewed by: kib, jhb (prior version)
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D15469
ff1fac3cb1