CURL-CVE-2023-27534

SFTP path ~ resolving discrepancy

curl supports SFTP transfers. curl's SFTP implementation offers a special feature in the path component of URLs: a tilde (`~`) character as the first path element in the path to denotes a path relative to the user's home directory. This is supported because of wording in the [once proposed to-become RFC draft](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-scp-sftp-ssh-uri-04) that was to dictate how SFTP URLs work. Due to a bug, the handling of the tilde in SFTP path did however not only replace it when it is used stand-alone as the first path element but also wrongly when used as a mere prefix in the first element. Using a path like `/~2/foo` when accessing a server using the user `dan` (with home directory `/home/dan`) would then quite surprisingly access the file `/home/dan2/foo`. This can be taken advantage of to circumvent filtering or worse.