libcurl: Restrict redirect schemes

All protocols except for CURLPROTO_FILE/CURLPROTO_SMB and their TLS
counterpart were allowed for redirect. This vastly broadens the
exploitation surface in case of a vulnerability such as SSRF [1], where
libcurl-based clients are forced to make requests to arbitrary hosts.

For instance, CURLPROTO_GOPHER can be used to smuggle any TCP-based
protocol by URL-encoding a payload in the URI. Gopher will open a TCP
connection and send the payload.

Only HTTP/HTTPS and FTP are allowed. All other protocols have to be
explicitly enabled for redirects through CURLOPT_REDIR_PROTOCOLS.

[1]: https://www.acunetix.com/blog/articles/server-side-request-forgery-vulnerability/

Signed-off-by: Linos Giannopoulos <lgian@skroutz.gr>

Closes #4094

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