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/basic/io-util.c

@@ -7,6 +7,7 @@
 
 #include "errno-util.h"
 #include "io-util.h"
+#include "iovec-util.h"
 #include "string-util.h"
 #include "time-util.h"
 
@@ -305,23 +306,6 @@ ssize_t sparse_write(int fd, const void *p, size_t sz, size_t run_length) {
         return q - (const uint8_t*) p;
 }
 
-char* set_iovec_string_field(struct iovec *iovec, size_t *n_iovec, const char *field, const char *value) {
-        char *x;
-
-        x = strjoin(field, value);
-        if (x)
-                iovec[(*n_iovec)++] = IOVEC_MAKE_STRING(x);
-        return x;
-}
-
-char* set_iovec_string_field_free(struct iovec *iovec, size_t *n_iovec, const char *field, char *value) {
-        char *x;
-
-        x = set_iovec_string_field(iovec, n_iovec, field, value);
-        free(value);
-        return x;
-}
-
 struct iovec_wrapper *iovw_new(void) {
         return malloc0(sizeof(struct iovec_wrapper));
 }
@@ -429,13 +413,3 @@ rollback:
         target->count = original_count;
         return r;
 }
-
-void iovec_array_free(struct iovec *iov, size_t n) {
-        if (!iov)
-                return;
-
-        for (size_t i = 0; i < n; i++)
-                free(iov[i].iov_base);
-
-        free(iov);
-}