MFC r315360:

MFC r315360:

Return NULL instead of 0 on failure in _kvm_open, kvm_open{,2,files}

This is being done for the following reasons:
- kvm_open(3), etc says they will return NULL.
- NULL by definition is (void*)0 per POSIX, but can be redefined,
  depending on the compiler, etc.

git-svn-id: svn://svn.freebsd.org/base/stable/10@315787 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f