sftp: merge the two uds creation paths

this was previously wildly duplicated. createUDSEntry has been refitted
to support both calls from stat() and from listDir().

createUDSEntry now takes an incoming SFTPAttributesPtr (taking ownership
of the attribute) and fills the udsentry accordingly. for listDir
efficiency's sake we pass in the entry by reference as well as the
details such that we can avoid needless work where possible.
the function now returns a conclusive Result, callers may naturally
ignore it.

listDir no longer aborts on readlink errors. indeed any errors coming
out of udsentry creation shouldn't impact listing. just because a file
fails to list doesn't mean the entire listing should stop 🤷

listDir now ignores readlink problems. there's no reason why failing to
read links would abort the dir listing