[dnsapi] Support adding / removing multiple TXT values for Gandi

[drott]

Gandi supports setting multiple entries by setting multiple array items
for the rrset_values field in their API. Modify the dns_gandi_livedns.sh
script so that it checks for existing entries, appends new ones if
needed, and removes existing ones individually. This enabled wildcard
certificate support on Gandi.

Fixes the dns_gandi_livedns part of #1261.

Tested by creating a multidomain, multiple wild-card certificate on
Gandi and using a test script executing only the dns_gandi_livedns_add
and dns_gandi_livedns_rm functions.

[drott]

CC @fcrozat

[drott]

The test script I was using for testing the add/rm functions individually was something like:

#!/usr/bin/env sh

. dns_gandi_livedns.sh
. ../acme.sh

DEBUG=2
GANDI_LIVEDNS_KEY=<removed>

#dns_gandi_livedns_add $1 $2
dns_gandi_livedns_rm $1 $2

[drott]

Updated and shfmt'ed.

[drott]

Fixed $_sub_domain vs $sub_domain issue, more formatting.

[Neilpang]

please send PR to dev branch, instead of master.

Thanks.

[drott]

please send PR to dev branch, instead of master.

Sure, sorry about that - I had seen the guidance in the commit msg and thought I had selected dev as base, but something must have slipped. Rebased on top of dev and base branch changed now.

[Neilpang]

thanks