- Disable warning C4127 "conditional expression is constant" globally
  in curl_setup.h for when building with Microsoft's compiler.

This mainly affects building with the Visual Studio project files found
in the projects dir.

Prior to this change the cmake and winbuild build systems already
disabled 4127 globally for when building with Microsoft's compiler.
Also, 4127 was already disabled for all build systems in the limited
circumstance of the WHILE_FALSE macro which disabled the warning
specifically for while(0). This commit removes the WHILE_FALSE macro and
all other cruft in favor of disabling globally in curl_setup.

Background:

We have various macros that cause 0 or 1 to be evaluated, which would
cause warning C4127 in Visual Studio. For example this causes it:

    #define Curl_resolver_asynch() 1

Full behavior is not clearly defined and inconsistent across versions.
However it is documented that since VS 2015 Update 3 Microsoft has
addressed this somewhat but not entirely, not warning on while(true) for
example.

Prior to this change some C4127 warnings occurred when I built with
Visual Studio using the generated projects in the projects dir.

Closes https://github.com/curl/curl/pull/4658

- no need to have them protocol specific

- no need to set pointers to them with the Curl_setup_transfer() call

- make Curl_setup_transfer() operate on a transfer pointer, not
  connection

- switch some counters from long to the more proper curl_off_t type

Closes #3627

The function does not return the same value as snprintf() normally does,
so readers may be mislead into thinking the code works differently than
it actually does. A different function name makes this easier to detect.

Reported-by: Tomas Hoger
Assisted-by: Daniel Gustafsson
Fixes #3296
Closes #3297

This enables level 4 instead of the default level 3, which of the
currently used comments only allows /* FALLTHROUGH */ to silence the
warning.

Closes https://github.com/curl/curl/pull/2747

On Windows, casting between unrelated function types is fine and
sometimes even necessary, so just use an intermediate cast to
(void (*) (void)) to silence the warning as described in [0].

[0] https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html

Closes https://github.com/curl/curl/pull/2860

Their usage was removed in 3a145180cc754a5959ca971ef3cd243c5c83fc51.

Closes #2852

telnet.c(1401,28): warning: cast from function call of type 'int' to
non-matching type 'HANDLE' (aka 'void *') [-Wbad-function-cast]

Fixes #2696
Closes #2700

- Get rid of variable that was generating false positive warning
(unitialized)

- Fix issues in tests

- Reduce scope of several variables all over

etc

Closes #2631

Detected using the `codespell` tool (version 1.13.0).

Also secure and fix an URL.

Closes #2623