... 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

Makes get_protocol_family() faster and it moves the knowledge about the
"families" to each protocol handler, where it belongs.

Closes #5986

... when continuing a transfer from a FETCH response.

When the size of the file was small enough that the entirety of the
transfer happens in a single go and schannel buffers holds the entire
data. However, it wasn't completely read in Curl_pp_readresp since a
line break was found before that could happen. So, by the time we are in
imap_state_fetch_resp - there's data in buffers that needs to be read
via Curl_read but nothing to read from the socket. After we setup a
transfer (Curl_setup_transfer), curl just waits on the socket state to
change - which doesn't happen since no new data ever comes.

Closes #5961

... or all "control codes" or nothing.

Assisted-by: Nicolas Sterchele

And remove a few unused booleans!

Closes #5461

Fixes warning detected by PVS-Studio
Fixes #4374

It was used (intended) to pass in the size of the 'socks' array that is
also passed to these functions, but was rarely actually checked/used and
the array is defined to a fixed size of MAX_SOCKSPEREASYHANDLE entries
that should be used instead.

Closes #4169

Reduce variable scopes and remove redundant variable stores.

Closes https://github.com/curl/curl/pull/3975

They serve very little purpose and mostly just add noise. Most of them
have been around for a very long time. I read them all before removing
or rephrasing them.

Ref: #3876
Closes #3883