Bug 11078: Add locking to rebuild_zebra

This patch adds locking to rebuild_zebra.pl to ensure that simultaneous
changes are prevented (as one is likely to overwrite the other).
Incremental updates in daemon mode will skipped if the lock is busy
and they will be picked up on the next pass.  Non-daemon mode
invocations will also exit immediately if they cannot get the lock
unless the new flag -wait-for-lock is specified, in which case they
will wait until the get the lock and then proceed.

Supporting changes made to Makefile.PL and templates for the new
locking directory (paralleling the other zebra lock directories).
We stash the zebra_lockdir in koha-conf.xml so rebuild_zebra.pl
can find it.

To address earlier QA concerns we:
1. added code to check if flock is available and ignore locking if
it's missing (from M. de Rooy)

2. changed default for adhoc invocations to abort if they cannot
obtain the lock.  Added option -wait-for-lock if the user prefers
to wait until the lock is free, and then continue processing.

3. added missing entry to t/db_dependent/zebra_config.pl

4. added a fallback locking directory of /tmp

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Doug merged the original patch with the QA changes.
Just for the record, noting here that the original patch was tested
extensively too by Martin Renvoize.
I have added a followup for some exceptional cases.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 11078: QA Follow-up for missing file permissions on lockfile

The original patch creates a lockfile in the ZEBRA_LOCKDIR.
It can fall back to /var/lock or even /tmp.
If the create fails, it dies. This can be considered as very
exceptional.

This followup adjusts the fallback location in /var/lock or /tmp
slightly.  It appends the database name to the folder in order to
prevent interfering between multiple Koha instances. Creation of the
lockfile has been moved to a subroutine extending directory and file
creation testing.

In the very unlikely case that we cannot create the lockfile (after
three separate tries), this follow-up allows you to continue instead
of die.  This is just as we did before we had file locking here. Every
time skipping a reindex could cause more harm than continuing and
having the race condition once in a while.

Test plan:
Test adding and removing lockdir from your koha-conf.xml. Check fallback.
Note that fallback in /var/lock or /tmp must contain database name.
Remove the lockdir config line and remove permissions from fallback. In
this case the reindex should continue but with a warning.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested with daemon and one-off invocation simultaneously.
Tested new wait parameter.
Tried all variations of lock directory (changing permissions etc.)

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 11078: (follow-up) tidy code

- fix a couple typos in comments
- make replace a "$i" with a more descriptive variable name
- style some of the new code

Signed-off-by: Galen Charlton <gmc@esilibrary.com>