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/core/exec-invoke.c

@@ -40,6 +40,7 @@
 #include "fd-util.h"
 #include "hexdecoct.h"
 #include "io-util.h"
+#include "iovec-util.h"
 #include "missing_ioprio.h"
 #include "missing_prctl.h"
 #include "missing_securebits.h"