io-util: split out "struct iovec" related calls into their own .c/.h files

This is preparation for #28891, which adds a bunch more helpers around
"struct iovec", at which point this really deserves its own .c/.h file.

The idea is that we sooner or later can consider "struct iovec" as an
entirely generic mechanism to reference some binary blob, and is the
go-to type for this purpose whenever we need one.

src/shared/ask-password-api.c

@@ -28,6 +28,7 @@
 #include "fs-util.h"
 #include "glyph-util.h"
 #include "io-util.h"
+#include "iovec-util.h"
 #include "keyring-util.h"
 #include "log.h"
 #include "macro.h"

src/shared/openssl-util.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "io-util.h"
+#include "iovec-util.h"
 #include "macro.h"
 #include "sha256.h"
 

src/shared/varlink.c

@@ -11,6 +11,7 @@
 #include "glyph-util.h"
 #include "hashmap.h"
 #include "io-util.h"
+#include "iovec-util.h"
 #include "list.h"
 #include "path-util.h"
 #include "process-util.h"