When using HTTPS proxy, SSL is used but not in the view of the FTP
protocol handler itself so separate the connection's use of SSL from the
FTP control connection's sue.

Fixes #5523

... reuses the same dynamic buffer instead of doing repeated malloc/free
cycles.

Test case 100 (FTP dir list PASV) does 7 fewer memory allocation calls
after this change in my test setup (132 => 125), curl 7.72.0 needed 140
calls for this.

Test case 103 makes 9 less allocations now (130). Down from 149 in
7.72.0.

Closes #6004

Closes #6004

... by introducing a printf() function that appends directly into a
dynbuf: Curl_dyn_vprintf(). This avoids the mandatory extra malloc so if
the buffer is already big enough it can just printf directly into it.

Since this less-malloc version requires tthe use of a library internal
printf function, we only provide this version when building libcurl and
not for the dynbuf code that is used when building the curl tool.

Closes #5998

Closes #5403

This typically makes 7-9 fewer mallocs per FTP transfer.

Closes #5997

The OS is deprecated. I see no traces of anyone having actually built
curl for Symbian after 2012.

The public headers are unmodified.

Closes #5989

Follow-up from f4873ebd0be32cf

Turns out some older openssl installations go bananas otherwise.
Reported-by: Tom van der Woerdt
Fixes #5995
Closes #5996