basic: define HEXDIGITS

define HEXDIGITS alongside DIGITS, and use it where it's already useful.  We'll
use it again shortly when parsing MAC addresses.

src/basic/string-util.h

@@ -37,6 +37,7 @@
 #define UPPERCASE_LETTERS "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 #define LETTERS           LOWERCASE_LETTERS UPPERCASE_LETTERS
 #define ALPHANUMERICAL    LETTERS DIGITS
+#define HEXDIGITS         DIGITS "abcdefABCDEF"
 
 #define streq(a,b) (strcmp((a),(b)) == 0)
 #define strneq(a, b, n) (strncmp((a), (b), (n)) == 0)

src/test/test-fileio.c

@@ -289,7 +289,7 @@ static void test_capeff(void) {
 
                 assert_se(r == 0);
                 assert_se(*capeff);
-                p = capeff[strspn(capeff, DIGITS "abcdefABCDEF")];
+                p = capeff[strspn(capeff, HEXDIGITS)];
                 assert_se(!p || isspace(p));
         }
 }