kern_exec: p_osrel and p_fctl0 were obliterated by failed execve(2) attempt.

Zeroing of them is needed so that an image activator can update the
values as appropriate (or not set at all).

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D22379