string-util: add new strdupcspn()/strdupspn()

These combine strndup() + strspn()/strcspn() into one.

There are a bunch of strndupa() calls that could use similar treatment
(or should be converted to strdup[c]spn(), but this commit doesn't
bother with that.

src/shared/discover-image.c

@@ -175,15 +175,13 @@ static int image_new(
 static int extract_pretty(const char *path, const char *suffix, char **ret) {
         _cleanup_free_ char *name = NULL;
         const char *p;
-        size_t n;
 
         assert(path);
         assert(ret);
 
         p = last_path_component(path);
-        n = strcspn(p, "/");
 
-        name = strndup(p, n);
+        name = strdupcspn(p, "/");
         if (!name)
                 return -ENOMEM;
 

src/shared/logs-show.c

@@ -108,7 +108,7 @@ static int url_from_catalog(sd_journal *j, char **ret) {
         weblink += strspn(weblink, " \t");
 
         /* Cut out till next whitespace/newline */
-        url = strndup(weblink, strcspn(weblink, WHITESPACE));
+        url = strdupcspn(weblink, WHITESPACE);
         if (!url)
                 return log_oom();