Bug 23400: Add --status to koha-indexer

Bug 23400: Add --status to koha-indexer

koha-zebra has a nice, informative --status switch, koha-indexer
does not. This patch adds it.

To test:
- Run "sudo koha-indexer --status kohadev" - you get an error:
  "Error: invalid option switch (--status)"
- Apply this patch
- If you are on e.g. kohadevbox you need to copy koha-indexer to
  where it is actually run from:
  sudo cp debian/scripts/koha-indexer /usr/sbin/koha-indexer
- Run "sudo koha-indexer --status kohadev" again and you should
  get a nice status telling if the indexer is running or not
- Toggle the indexer on and off with koha-indexer --start/--stop
  and verify the status is telling the truth about the state of
  the indexer
- Run "sudo koha-indexer --help" and check there is a description
  of the --status option
- Have a look at debian/docs/koha-indexer.xml and verify there is
  an adequate description of the --status option.
- Run "prove -v xt/verify-debian-docbook.t" to check the structure
  of the man page in DocBook format.

PS: I tried to generate a manpage from the DocBook as described at
https://wiki.koha-community.org/wiki/Testing_man_pages
but the output did not look good for any of our DocBook files.
Not sure why.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>