MFC r300792,r300851,r301580:

MFC r300792,r300851,r301580:

  r300792:
    exec: Add credential change information into imgp for process_exec hook.
  r300851:
    exec: get rid of one vnode lock/unlock pair in do_execve
  r301580:
    Old process credentials for setuid execve must not be dereferenced when the
    process credentials were not changed.  This can happen if an error occured
    trying to activate the setuid binary.  And on error, if new credentials
    were not yet assigned, they must be freed to not create the leak.

git-svn-id: svn://svn.freebsd.org/base/stable/10@302238 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f