MFC 213672,213674,214396:

MFC 213672,213674,214396:
- Report subcommand handler errors in mfiutil so that tools that
  invoke the utilities can robustly report errors.
- Fix compile with -DDEBUG by using the correct mfi_pd_ref union definition
  in mfireg.h.
- Save errno values before calling warn(3) so that errors are correctly
  reported.
- Use powerof2() from <sys/param.h> rather than a copy and paste version.

git-svn-id: svn://svn.freebsd.org/base/stable/8@214742 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f