Closes #5968

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

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

Assisted-by: Nicolas Sterchele

Text 1560 is extended to verify.

Reported-by: Pavel Volgarev
Fixes #5344
Closes #5351

In the "scheme-less" parsing case, we need to strip off credentials
first before we guess scheme based on the host name!

Assisted-by: Jay Satiro
Fixes #4856
Closes #4857

Follow-up from 2c20109a9b5d04

Added test 663 to verify.

Reported by OSS-Fuzz
Bug: https://crbug.com/oss-fuzz/17954

Closes #4453

`dest` is only used with `ENABLE_IPV6`.

Closes https://github.com/curl/curl/pull/4444

The parser would check for a query part before fragment, which caused it
to do wrong when the fragment contains a question mark.

Extended test 1560 to verify.

Reported-by: Alex Konev
Fixes #4412
Closes #4413

If the input hostname is "[", hlen will underflow to max of size_t when
it is subtracted with 2.

hostname[hlen] will then cause a warning by ubsanitizer:

runtime error: addition of unsigned offset to 0x<snip> overflowed to
0x<snip>

I think that in practice, the generated code will work, and the output
of hostname[hlen] will be the first character "[".

This can be demonstrated by the following program (tested in both clang
and gcc, with -O3)

int main() {
  char* hostname=strdup("[");
  size_t hlen = strlen(hostname);

  hlen-=2;
  hostname++;
  printf("character is %d\n",+hostname[hlen]);
  free(hostname-1);
}

I found this through fuzzing, and even if it seems harmless, the proper
thing is to return early with an error.

Closes #4389