chasquid-util: Remove dependency on docopt-go

The docopt-go library is quite convenient, but it has been abandoned for
a while :(

Since we only use it for chasquid-util, this patch removes it and
replaces it with a custom small parser, that is a reasonable fit for the
required use cases.

The patch also adds a couple of tests to increase coverage.

NOTE: docopt-go accepted some undocumented behaviour, in particular the
use of "-a b" instead of "-a=b". The new parser does not, so some
user scripts may require updating.

I think this should be rare enough not to be worth the complexity of
adjusting the parser to allow it.

test/util/lib.sh

@@ -65,9 +65,9 @@ function add_user() {
 	DOMAIN=$(echo $1 | cut -d @ -f 2)
 	mkdir -p "${CONFDIR}/domains/$DOMAIN/"
 	go run ${TBASE}/../../cmd/chasquid-util/chasquid-util.go \
-		-C "${CONFDIR}" \
+		-C="${CONFDIR}" \
 		user-add "$1" \
-		--password "$2" \
+		--password="$2" \
 		>> .add_user_logs
 }