dhcp-client: parse RFC8910 captive portal dhcp option

src/basic/string-util.h

@@ -22,6 +22,9 @@
 #define ALPHANUMERICAL      LETTERS DIGITS
 #define HEXDIGITS           DIGITS "abcdefABCDEF"
 #define LOWERCASE_HEXDIGITS DIGITS "abcdef"
+#define URI_RESERVED        ":/?#[]@!$&'()*+;="         /* [RFC3986] */
+#define URI_UNRESERVED      ALPHANUMERICAL "-._~"       /* [RFC3986] */
+#define URI_VALID           URI_RESERVED URI_UNRESERVED /* [RFC3986] */
 
 static inline char* strstr_ptr(const char *haystack, const char *needle) {
         if (!haystack || !needle)