std.c reorganization

It is now composed of these main sections:
* Declarations that are shared among all operating systems.
* Declarations that have the same name, but different type signatures
  depending on the operating system. Often multiple operating systems
  share the same type signatures however.
* Declarations that are specific to a single operating system.
  - These are imported one per line so you can see where they come from,
    protected by a comptime block to prevent accessing the wrong one.

Closes #19352 by changing the convention to making types `void` and
functions `{}`, so that it becomes possible to update `@hasDecl` sites
to use `@TypeOf(f) != void` or `T != void`. Happily, this ended up
removing some duplicate logic and update some bitrotted feature
detection checks.

A handful of types have been modified to gain namespacing and type
safety. This is a breaking change.

Oh, and the last usage of `usingnamespace` site is eliminated.

lib/std/process.zig

@@ -1789,10 +1789,7 @@ pub fn cleanExit() void {
 /// On some systems, this raises the limit before seeing ProcessFdQuotaExceeded
 /// errors. On other systems, this does nothing.
 pub fn raiseFileDescriptorLimit() void {
-    const have_rlimit = switch (native_os) {
-        .windows, .wasi => false,
-        else => true,
-    };
+    const have_rlimit = posix.rlimit_resource != void;
     if (!have_rlimit) return;
 
     var lim = posix.getrlimit(.NOFILE) catch return; // Oh well; we tried.