lib/config, lib/fs: Make junction behaviour configurable (ref #6606)

[imsodin]

See https://forum.syncthing.net/t/option-to-follow-directory-junctions-symbolic-links/14750/126 and following.

[AudriusButkevicius]

I think we do a config migration of this, setting to true on all existing folders, and defaulting to false.

[imsodin]

Passing fs options as url-like query parameters might not be the best idea: A filesystem path does not necessarily conform to what url.Parse expects. An alternative is adding the usual ...opts Option thing to NewFilesystem and then have WithJunctionsAsDirs() Option, though I believe Audrius has a bit of a disliking to that pattern (?).

[AudriusButkevicius]

I don't mind options, I mind functions thrown around like they are variables, yet I think query string stuff is the right way to go here.

[imsodin]

Well these options are functions disguised as options ;)

Anyway my problem is that almost anything is allowed in paths, i.e. you need to do parsing yourself (url.Parse fails). At which point I'd rather not do the detour of encoding options into a pseudo-uri and then decoding the same options from that pseudo-uri, but pass them directly.

[AudriusButkevicius]

so my original idea was to turn C:\foo\bar into file://c/foo/bar, but it never happened. If it did happen, this would be much easier.

This also would have allowed getting rid fo the type, as the schema dictates the type.

[imsodin]

That makes sense, but being an unrelated change and quite possible to turn up weird cornercases, I used options, It's still easy enough to switch to "all uri initialisation" from here.

[calmh]

Panic if I ask for a fakefs with junctionsAsDirs?