Handle kd == NULL gracefully with kvm_close(3)

[ngie]

Summary

Handle kd == NULL gracefully with kvm_close(3)

Don't segfault in kvm_close(3) if provided a NULL pointer. Instead, return
-1 and set errno to EINVAL.

Document this new behavior explicitly.

MFC after: 1 week
Sponsored by: Dell EMC Isilon

[ngie]

Add kvm_open(3) manpage changes (oddly, arc diff --create didn't complain
about the second arg being invalid -- hah!)

[ngie]

lib/libkvm/kvm_open.3

247 ↗

(On Diff #26459)

This needs to be changed

[ngie]

Remove * before description of kd argument passed to kvm_close in the ERRORS section