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/basic/fileio.c

@@ -862,7 +862,6 @@ int executable_is_script(const char *path, char **interpreter) {
 int get_proc_field(const char *filename, const char *pattern, const char *terminator, char **field) {
         _cleanup_free_ char *status = NULL;
         char *t, *f;
-        size_t len;
         int r;
 
         assert(terminator);
@@ -914,9 +913,7 @@ int get_proc_field(const char *filename, const char *pattern, const char *termin
                         t--;
         }
 
-        len = strcspn(t, terminator);
-
-        f = strndup(t, len);
+        f = strdupcspn(t, terminator);
         if (!f)
                 return -ENOMEM;
 

src/basic/hostname-util.c

@@ -62,7 +62,7 @@ int gethostname_full(GetHostnameFlags flags, char **ret) {
         }
 
         if (FLAGS_SET(flags, GET_HOSTNAME_SHORT))
-                buf = strndup(s, strcspn(s, "."));
+                buf = strdupcspn(s, ".");
         else
                 buf = strdup(s);
         if (!buf)

src/basic/string-util.c

@@ -1201,3 +1201,19 @@ size_t strspn_from_end(const char *str, const char *accept) {
 
         return n;
 }
+
+char *strdupspn(const char *a, const char *accept) {
+        if (isempty(a) || isempty(accept))
+                return strdup("");
+
+        return strndup(a, strspn(a, accept));
+}
+
+char *strdupcspn(const char *a, const char *reject) {
+        if (isempty(a))
+                return strdup("");
+        if (isempty(reject))
+                return strdup(a);
+
+        return strndup(a, strcspn(a, reject));
+}

src/basic/string-util.h

@@ -240,3 +240,6 @@ bool streq_skip_trailing_chars(const char *s1, const char *s2, const char *ok);
 char *string_replace_char(char *str, char old_char, char new_char);
 
 size_t strspn_from_end(const char *str, const char *accept);
+
+char *strdupspn(const char *a, const char *accept);
+char *strdupcspn(const char *a, const char *reject);