Bug 21440: koha-create expects the file passed by $DEFAULTSQL to be in gzip format

Bug 21440: koha-create expects the file passed by $DEFAULTSQL to be in gzip format

koha-create contains a call to `zcat` which fails if the file passed
as an argument is not in gzip format.

This patch fixes the issue by adding the -f flag to the zcat call in
koha-create, as per the top-voted answers in the following SE URLs:
https://unix.stackexchange.com/a/77309
https://unix.stackexchange.com/a/131944

Test plan:
1) in /etc/koha/koha-sites.conf, set the DEFAULTSQL variable to point
   to an _uncompressed_ SQL database dump
2) run `koha-create --create-db instance` and notice how it fails with:
   gzip: /path/to/dump.sql: not in gzip format
3) apply the patch
4) notice the extra line of information "The SQL file can be optionally
   compressed with gzip" in koha-sites.conf
5) run the `koha-create` command again, this time it should work
6) repeat with DEFAULTSQL pointing to a gzip'd dump, it should work too

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>