Bug 7924 - Fix handling of command line arguments in koha-remove

Bug 7924 - Fix handling of command line arguments in koha-remove

koha-remove would fail with a message like this:
Removing Koha instance --
ERROR 1396 (HY000) at line 1: Operation DROP USER failed for 'koha_--'@'%'

After fixing this, the --keep-mysql option made the script exit without doing
anything.

To test:

Alt 1
- Build package with build-git-snapshot
- Install package

Alt 2
- Replicate the changes in the patch in an existing installation

Then:
- Add instance1 and instance2
- koha-remove instance1
-- Check that script completes and the databse is gone
- koha-remove --keep-mysql instance2
-- Check that the script completes but the database is not removed

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Cherry-picked to 3.6.x prior to inclusion in Master because this is required for
the release build.