Change all DBUS_TYPE_G_MAP_OF_STRING properties to G_TYPE_HASH_TABLE,
with annotations indicating they are string->string. Not much outside
libnm-core needs to changed for this, since DBUS_TYPE_G_MAP_OF_STRING
was already represented as a hash table.

(One change needed within libnm-core is that we now need to copy the
hash tables in get_property(), or else the caller will receive a
reffed copy of the object's own hash table, which we don't want.)