smb: reshuffle discovery systems

the modern discoveries were conditional on smbc_opendir succeeding but
that is rather silly since smbc can fail on ENOENT if (for example)
netbios is disabled. since top level server discovery may happen through
all or none of NB/DNSSD/WSD that made no sense as even when NB fails
we may be able to produce service listing through one of the other two
discovery services.

this piece of code would actually benefit from major refactoring because
all three discovery systems should actually run at the same time while
currently NB holds up the entire show. alas, too invasive for 20.04...
so, reshuffling it is:

smbc_opendir still runs initially. after that, if the url is browsing
smb:// we'll jump into modern discovery. any errors produced by opendir
will be ignored! authentication errors and the like cannot happen for
smb:// because we aren't yet talking to any one server, which leaves
actual browsing errors which are not fatal considering we have other
options.
for non-top-level urls we'll otherwise jump into error handling (e.g.
server requires auth or something)
at the end if we haven't returned early with an error we'll inject our
`.` entries as per usual

test plan:
- add `disable netbios = yes` to smb.conf
- browse network
- discovers dnssd/wsd servers
- also works with the option removed again

BUG: 421624

FIXED-IN: 20.04.2