Follow-up from f4873ebd0be32cf

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

These two files were always tightly connected and it was hard to
understand what went into which. This also allows us to make the
ftpsend() function static (moved from ftp.c).

Removed security.c
Renamed curl_sec.h to krb5.h

Closes #5987

 const struct curl_easyoption *curl_easy_option_by_name(const char *name);

 const struct curl_easyoption *curl_easy_option_by_id (CURLoption id);

 const struct curl_easyoption *
 curl_easy_option_next(const struct curl_easyoption *prev);

The purpose is to provide detailed enough information to allow for
example libcurl bindings to get option information at run-time about
what easy options that exist and what arguments they expect.

Assisted-by: Jeroen Ooms
Closes #5365

Closes https://github.com/curl/curl/pull/5754

Create a set of routines for TLS key log file handling to enable reuse
with other TLS backends. Simplify the OpenSSL backend as follows:

 - Drop the ENABLE_SSLKEYLOGFILE macro as it is unconditionally enabled.
 - Do not perform dynamic memory allocation when preparing a log entry.
   Unless the TLS specifications change we can suffice with a reasonable
   fixed-size buffer.
 - Simplify state tracking when SSL_CTX_set_keylog_callback is
   unavailable. My original sslkeylog.c code included this tracking in
   order to handle multiple calls to SSL_connect and detect new keys
   after renegotiation (via SSL_read/SSL_write). For curl however we can
   be sure that a single master secret eventually becomes available
   after SSL_connect, so a simple flag is sufficient. An alternative to
   the flag is examining SSL_state(), but this seems more complex and is
   not pursued. Capturing keys after server renegotiation was already
   unsupported in curl and remains unsupported.

Tested with curl built against OpenSSL 0.9.8zh, 1.0.2u, and 1.1.1f
(`SSLKEYLOGFILE=keys.txt curl -vkso /dev/null https://localhost:4433`)
against an OpenSSL 1.1.1f server configured with:

    # Force non-TLSv1.3, use TLSv1.0 since 0.9.8 fails with 1.1 or 1.2
    openssl s_server -www -tls1
    # Likewise, but fail the server handshake.
    openssl s_server -www -tls1 -Verify 2
    # TLS 1.3 test. No need to test the failing server handshake.
    openssl s_server -www -tls1_3

Verify that all secrets (1 for TLS 1.0, 4 for TLS 1.3) are correctly
written using Wireshark. For the first and third case, expect four
matches per connection (decrypted Server Finished, Client Finished, HTTP
Request, HTTP Response). For the second case where the handshake fails,
expect a decrypted Server Finished only.

    tshark -i lo -pf tcp -otls.keylog_file:keys.txt -Tfields \
        -eframe.number -eframe.time -etcp.stream -e_ws.col.Info \
        -dtls.port==4433,http -ohttp.desegment_body:FALSE \
        -Y 'tls.handshake.verify_data or http'

A single connection can easily be identified via the `tcp.stream` field.

If the QLOGDIR environment variable is set, enable qlogging.

... and create Curl_qlogdir() in the new generic vquic/vquic.c file for
QUIC functions that are backend independent.

Closes #5353

A common set of functions instead of many separate implementations for
creating buffers that can grow when appending data to them. Existing
functionality has been ported over.

In my early basic testing, the total number of allocations seem at
roughly the same amount as before, possibly a few less.

See docs/DYNBUF.md for a description of the API.

Closes #5300

Closes #5173

And make the cookie save function use it.

As detailed in DEPRECATE.md, the polarssl support is now removed after
having been disabled for 6 months and nobody has missed it.

The threadlock files used by mbedtls are renamed to an 'mbedtls' prefix
instead of the former 'polarssl' and the common functions that
previously were shared between mbedtls and polarssl and contained the
name 'polarssl' have now all been renamed to instead say 'mbedtls'.

Closes #4825