MSVC: Improve warning options set

meson.build has a list of warnings to disable on MSVC:

cflags_warn += [
'/wd4018', # signed/unsigned mismatch
'/wd4244', # conversion from 'type1' to 'type2', possible loss of data
'/wd4273', # inconsistent DLL linkage
'/wd4101', # unreferenced local variable
'/wd4102', # unreferenced label
'/wd4090', # different 'modifier' qualifiers
'/wd4267', # conversion from 'size_t' to 'type', possible loss of data
]

First, these appear to be in some random order, so I wanted to sort
them. But then it also appeared that for some of these, if you remove
the disablement, nothing changes, so it seemed some of these entries are
not needed.

Some of these warnings are assigned to higher warning levels, so if
someone wanted to compile PostgreSQL with a higher warning level on MSVC
(similar to -Wextra), then disabling some from the higher levels is
useful. But I figured this could be explained better.

So what I did is sort these and group them by the warning level assigned
by MSVC.

Actually, one of them (the "unreferenced label" one) was not enabled by
default on any level, and conversely I think we do actually want that
one enabled, since we use -Wunused-label on other compilers, so I
changed the disablement to an enablement.

Then I also found a few more warnings that would be useful to enable.
So I'm proposing to add warnings that are similar to -Wformat and -Wswitch.

Here are some documentation links:

https://learn.microsoft.com/en-us/cpp/build/reference/compiler-option-warning-level?view=msvc-170
https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warnings-c4000-c5999?view=msvc-170
https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warnings-by-compiler-version?view=msvc-170

With that done, I also looked at the disabled warnings to see what could
be done to fix the underlying issues. In particular, I looked at

'/wd4273', # inconsistent DLL linkage

If you enable that one, you get this:

../src/backend/utils/misc/ps_status.c(27): warning C4273:
'__p__environ': inconsistent dll linkage
C:\Program Files (x86)\Windows
Kits\10\include\10.0.22621.0\ucrt\stdlib.h(1158): note: see previous
definition of '__p__environ'

The declaration in ps_status.c was:

#if !defined(WIN32) || defined(_MSC_VER)
extern char **environ;
#endif

The declaration in the OS header file is:

_DCRTIMP char*** __cdecl __p__environ (void);
#define _environ (*__p__environ())

So it is clear why a linker might be upset about this.

To fix this, we can just remove the || defined(_MSC_VER).

Note that these conditionals around the environ declarations were added
only somewhat recently in commit 7bc9a8bdd2d ("Fix warnings about
declaration of environ on MinGW.").

Maybe there are older versions of Windows where the declaration is
needed, maybe this is a ucrt vs. msvcrt thing, don't know, testing is
welcome.

The business in ps_status.c is kind of weird anyway, because we only
need the environ variable in the PS_USE_CLOBBER_ARGV case, which is not
Windows, so maybe we should move some things around to avoid the problem
in this file. But there are also a few other files where environ is
declared for use by Windows as well.