Low: tools: Fix scope/xpath parsing in cibadmin

Currently, if --xpath is specified and then --scope is specified, the
--scope optarg replaces the --xpath optarg, but the cib_xpath call
option remains set.

Since the behavior was already buggy in this situation, let's use this
as an opportunity to make the --xpath/--scope pair of arguments behave
in a last-wins manner.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>