MFC 216823, 223991, 223992:

MFC 216823, 223991, 223992:

216823 (by pjd):
- For compatibility with Linux and Solaris add poweroff(8).
  It is implemented as a hard link to shutdown(8) and it is equivalent
  of:

        # shutdown -p now

  While I'm here put one line of usage into one line of C code so it
  is easier to grep(1) and separate unrelated code with empty line.

223991:
- Improvements to the shutdown(8) manual.

223992:
- Remove trailing whitespace in the shutdown(8) manual.

Approved by:		delphij

git-svn-id: svn://svn.freebsd.org/base/stable/8@224259 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f