Do not leak oldvmspace if image activation failed

and current address space is already destroyed, so kern_execve()
terminates the process.

While there, clean up some internals of post_execve() inlined in init_main.

Reported by: Peter <pmc@citylink.dinoex.sub.org>
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D26525