mongo-utils: Allow -1 as port in mongo_util_parse_addr()

When using replica-sets with unix domain sockets, we end up putting
paths with -1 ports into the internal seed list. mongo_connect()
understands that, but mongo_util_parse_addr() needs to handle that
similarly too. To this end, allow -1 (MONGO_CONN_LOCAL) when parsing
addresses.

This really fixes #24.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>