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

Closes #5986

Provide the HTTP method that was used on the latest request, which might
be relevant for users when there was one or more redirects involved.

Closes #5511

Updated terminology in docs, comments and phrases to refer to C strings
as "null-terminated". Done to unify with how most other C oriented docs
refer of them and what users in general seem to prefer (based on a
single highly unscientific poll on twitter).

Reported-by: coinhubs on github
Fixes #5598
Closes #5608

Regression in 7.71.0

Added test case 346 to verify.

Reported-by: Kristoffer Gleditsch
Fixes #5613
Closes #5616

Reported-by: Jon Johnson Jr
Fixes #5582
Closes #5592

Since the connection can be used by many independent requests (using
HTTP/2 or HTTP/3), things like user-agent and other transfer-specific
data MUST NOT be kept connection oriented as it could lead to requests
getting the wrong string for their requests. This struct data was
lingering like this due to old HTTP1 legacy thinking where it didn't
mattered..

Fixes #5566
Closes #5567

Instead of discussing if there's value or meaning (implied or not) in
the colors, let's use words without the same possibly negative
associations.

Closes #5546

When the method is updated inside libcurl we must still not change the
method as set by the user as then repeated transfers with that same
handle might not execute the same operation anymore!

This fixes the libcurl part of #5462

Test 1633 added to verify.

Closes #5499

Added build to travis to verify

Closes #5466

And remove a few unused booleans!

Closes #5461