tree-wide: add global ascii_isdigit() + ascii_isalpha()

We now have a local implementation in string-util-fundamental.c, but
it's useful at a lot of other places, hence let's give it a more
expressive name and share it across the tree.

Follow-up for: 8d9156660d

src/shared/calendarspec.c

@@ -521,7 +521,7 @@ static int parse_component_decimal(const char **p, bool usec, int *res) {
         const char *e = NULL;
         int r;
 
-        if (!isdigit(**p))
+        if (!ascii_isdigit(**p))
                 return -EINVAL;
 
         r = parse_one_number(*p, &e, &value);

src/shared/device-nodes.c

@@ -5,13 +5,13 @@
 #include <string.h>
 
 #include "device-nodes.h"
+#include "string-util.h"
 #include "utf8.h"
 
 int allow_listed_char_for_devnode(char c, const char *white) {
         return
-                (c >= '0' && c <= '9') ||
-                (c >= 'A' && c <= 'Z') ||
-                (c >= 'a' && c <= 'z') ||
+                ascii_isdigit(c) ||
+                ascii_isalpha(c) ||
                 strchr("#+-.:=@_", c) ||
                 (white && strchr(white, c));
 }

src/shared/dissect-image.c

@@ -251,7 +251,7 @@ static int make_partition_devname(
         if (isempty(whole_devname)) /* Make sure there *is* a last char */
                 return -EINVAL;
 
-        need_p = strchr(DIGITS, whole_devname[strlen(whole_devname)-1]); /* Last char a digit? */
+        need_p = ascii_isdigit(whole_devname[strlen(whole_devname)-1]); /* Last char a digit? */
 
         return asprintf(ret, "%s%s%i", whole_devname, need_p ? "p" : "", nr);
 }

src/shared/dns-domain.c

@@ -236,9 +236,8 @@ int dns_label_escape(const char *p, size_t l, char *dest, size_t sz) {
                         sz -= 2;
 
                 } else if (IN_SET(*p, '_', '-') ||
-                           (*p >= '0' && *p <= '9') ||
-                           (*p >= 'a' && *p <= 'z') ||
-                           (*p >= 'A' && *p <= 'Z')) {
+                           ascii_isdigit(*p) ||
+                           ascii_isalpha(*p)) {
 
                         /* Proper character */
 
@@ -907,15 +906,13 @@ static bool srv_type_label_is_valid(const char *label, size_t n) {
                 return false;
 
         /* Second char must be a letter */
-        if (!(label[1] >= 'A' && label[1] <= 'Z') &&
-            !(label[1] >= 'a' && label[1] <= 'z'))
+        if (!ascii_isalpha(label[1]))
                 return false;
 
         /* Third and further chars must be alphanumeric or a hyphen */
         for (k = 2; k < n; k++) {
-                if (!(label[k] >= 'A' && label[k] <= 'Z') &&
-                    !(label[k] >= 'a' && label[k] <= 'z') &&
-                    !(label[k] >= '0' && label[k] <= '9') &&
+                if (!ascii_isalpha(label[k]) &&
+                    !ascii_isdigit(label[k]) &&
                     label[k] != '-')
                         return false;
         }