use isxdigit and apps_tohex

Replace ad-hoc ascii->hex with isxdigit and new app_tohex.

Reviewed-by: Andy Polyakov <appro@openssl.org>

apps/apps.c

@@ -2673,6 +2673,45 @@ int app_access(const char* name, int flag)
 #endif
 }
 
+int app_hex(char c)
+{
+    switch (c) {
+    default:
+    case '0':
+        return 0;
+    case '1':
+        return 1;
+    case '2':
+        return 2;
+    case '3':
+        return 3;
+    case '4':
+          return 4;
+    case '5':
+          return 5;
+    case '6':
+          return 6;
+    case '7':
+          return 7;
+    case '8':
+          return 8;
+    case '9':
+          return 9;
+    case 'a': case 'A':
+          return 0x0A;
+    case 'b': case 'B':
+          return 0x0B;
+    case 'c': case 'C':
+          return 0x0C;
+    case 'd': case 'D':
+          return 0x0D;
+    case 'e': case 'E':
+          return 0x0E;
+    case 'f': case 'F':
+          return 0x0F;
+    }
+}
+
 /* app_isdir section */
 #ifdef _WIN32
 int app_isdir(const char *name)