coredump: rename set_iovec_field_free() into set_iovec_string_field_free()

It's more in line with its counterpart set_iovec_string_field().

Also move the definition to io-util next to set_iovec_string_field().

src/basic/io-util.c

@@ -262,3 +262,11 @@ char* set_iovec_string_field(struct iovec *iovec, size_t *n_iovec, const char *f
                 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;
+}

src/basic/io-util.h

@@ -73,3 +73,4 @@ static inline bool FILE_SIZE_VALID_OR_INFINITY(uint64_t l) {
 #define IOVEC_MAKE_STRING(string) (struct iovec) IOVEC_INIT_STRING(string)
 
 char* set_iovec_string_field(struct iovec *iovec, size_t *n_iovec, const char *field, const char *value);
+char* set_iovec_string_field_free(struct iovec *iovec, size_t *n_iovec, const char *field, char *value);