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

Closes #5537

 - Stick to a single unified way to use structs
 - Make checksrc complain on 'typedef struct {'
 - Allow them in tests, public headers and examples

 - Let MD4_CTX, MD5_CTX, and SHA256_CTX typedefs remain as they actually
   typedef different types/structs depending on build conditions.

Closes #5338

Fine: "struct hello *world"

Not fine: "struct hello* world" (and variations)

Closes #5386

New test 669 checks this fix is effective.

Fixes #5256
Closes #5258
Reported-by: thanhchungbtc on github

New test 666 checks this is effective.
As upload buffer size is significant in this kind of tests, shorten it
in similar test 652.

Fixes #4860
Closes #4833
Reported-by: RuurdBeerstra on github

Input buffer filling may delay the data sending if data reads are slow.
To overcome this problem, file and callback data reads do not accumulate
in buffer anymore. All other data (memory data and mime framing) are
considered as fast and still concatenated in buffer.
As this may highly impact performance in terms of data overhead, an early
end of part data check is added to spare a read call.
When encoding a part's data, an encoder may require more bytes than made
available by a single read. In this case, the above rule does not apply
and reads are performed until the encoder is able to deliver some data.

Tests 643, 644, 645, 650 and 654 have been adapted to the output data
changes, with test data size reduced to avoid the boredom of long lists of
1-byte chunks in verification data.
New test 667 checks mimepost using single-byte read callback with encoder.
New test 668 checks the end of part data early detection.

Fixes #4826
Reported-by: MrdUkk on github

In case a read callback returns a status (pause, abort, eof,
error) instead of a byte count, drain the bytes read so far but
remember this status for further processing.
Takes care of not losing data when pausing, and properly resume a
paused mime structure when requested.
New tests 670-673 check unpausing cases, with easy or multi
interface and mime or form api.

Fixes #4813
Reported-by: MrdUkk on github

Due to the merge/revert cycle

This reverts commit 87869e38d7afdec3ef1bb4965711458b088e254f.

Fixes #5014
Closes #5015
Reopens #4833