- use `wmain` instead of `main` when `_UNICODE` is defined [0]
- define `argv_item_t` as `wchar_t *` in this case
- use the curl_multibyte gear to convert the command-line arguments to
  UTF-8

This makes it possible to pass parameters with characters outside of
the current locale on Windows, which is required for some tests, e.g.
the IDN tests. Out of the box, this currently only works with the
Visual Studio project files, which default to Unicode, and winbuild
with the `ENABLE_UNICODE` option.

[0] https://devblogs.microsoft.com/oldnewthing/?p=40643

Ref: https://github.com/curl/curl/issues/3747
Closes https://github.com/curl/curl/pull/3784

- Undefine DEBUGASSERT in curl_setup_once.h in case it was already
  defined as a system macro.

- Don't compile write32_le in curl_endian unless
  CURL_SIZEOF_CURL_OFF_T > 4, since it's only used by Curl_write64_le.

- Include <arpa/inet.h> in socketpair.c.

Closes https://github.com/curl/curl/pull/4756

Follow-up to 9c1806ae

- 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

- replace tabs with spaces where possible
- remove line ending spaces
- remove double/triple newlines at EOF
- fix a non-UTF-8 character
- cleanup a few indentations/line continuations
  in manual examples

Closes https://github.com/curl/curl/pull/3037

... since the libc provided one are locale dependent in a way we don't
want. Also, the "native" isalnum() (for example) works differently on
different platforms which caused test 1307 failures on macos only.

Closes #2269

Prior to this change (SET_)ERRNO mapped to GetLastError/SetLastError
for Win32 and regular errno otherwise.

I reviewed the code and found no justifiable reason for conflating errno
on WIN32 with GetLastError/SetLastError. All Win32 CRTs support errno,
and any Win32 multithreaded CRT supports thread-local errno.

Fixes https://github.com/curl/curl/issues/895
Closes https://github.com/curl/curl/pull/1589

SEND_QUAL_ARG2 had to be set, but was never used. Use it in swrite to
avoid warnings about casting away low-level const.

Closes https://github.com/curl/curl/pull/1464

lib/curl_setup_once.h assumed lwIP on Windows uses 'SetLastError()' to
set network errors. It doesn't; it uses 'errno'.