cookie: fix crash in netscape cookie parsing

- Parse the input string without modifying it.

Prior to this change a segfault could occur if the input string was
const because the tokenizer modified the input string. For example if
the user set CURLOPT_COOKIELIST to a const string then libcurl would
likely cause a crash when modifying that string. Even if the string was
not const or a crash did not occur there was still the incorrect and
unexpected modification of the user's input string.

This issue was caused by 30da1f59 (precedes 8.11.0) which refactored
some options parsing and eliminated the copy of the input string. Also,
an earlier commit f88cc654 incorrectly cast the input pointer when
passing it to strtok.

Co-authored-by: Daniel Stenberg

Closes https://github.com/curl/curl/pull/15826

docs/libcurl/opts/CURLOPT_COOKIELIST.md

@@ -81,7 +81,7 @@ NULL
 
 int main(void)
 {
-  char *my_cookie =
+  const char *my_cookie =
     "example.com"    /* Hostname */
     SEP "FALSE"      /* Include subdomains */
     SEP "/"          /* Path */