Allow single DA if explicitly configured
When on a private network a single directory authority is explicitly
configured, allow that DA's instace of tor to pick itself as its own
DA, by having router_pick_dirserver_generic() set the PDS_ALLOW_SELF
flag when the list of potential sources contains only one element.
This would subsequently allow router_pick_trusteddirserver_impl() to
calculate is 'requreother' condition as 'false', enabling it to pick
itself as a valid DA candidate.
Fixes #25928
Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>