Use bzero instead of explicitly zeroing stuff in do_execve.

While strictly speaking this is not correct since some fields are pointers,
it makes no difference on all supported archs and we already rely on it doing
the right thing in other places.

No functional changes.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272290 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f