powerd.8: Improve style & fix typos

[0mp]

Summary

powerd.8: Improve style & fix typos

- Sort options.
- Do not use macros (like .Ar) to specify width for Bl (macros within that
  string are not expanded).
- Use Cm instead of Ar for mode names.
- Fix some typos reported by mandoc.
- Move the documentation of the the PID file from the -P flag description to
  the FILES section.

[bcr]

Good to go. Thanks!

[0mp]

In D23941#526158, @bcr wrote:

Good to go. Thanks!

Thank you!