lib: new easy option string storage

Change the storage of easy handle option strings from an array sized for
all possible options to a hash set to reduce memory footprint.

Give the hash set initially room for 4 strings, with first allocation
happening when it goes beyond that. Measurements without test suite
and a forced fail on growing the set gives:

Size Result
2    1261 tests out of 1951 reported OK: 64%
4    1792 tests out of 1951 reported OK: 91%
8    1944 tests out of 1951 reported OK: 99%
16   1949 tests out of 1951 reported OK: 99%
32   single fail of 3211, unit test for u8_strset

Add u8_strset that keeps the tuples (uint8_t id, char *str)
and allows set/unset by `id`. Add that as data->set.strings.

Define MACROS
* CURL_EASY_STR(data, id) for access
* CURL_EASY_STR_SET(data, id, s) for setting, making a copy
* CURL_EASY_STR_SETN(data, id, s) for setting, no copy
* CURL_EASY_STR_CLEAR(data, id) for unsetting
* CURL_EASY_STR_CLEAR0(data, id) for unsetting and zero-ing value

Add `data->set.str_copypostfields` to handle former `STRING_COPYPOSTFIELDS`
string that was not always a string and could carry NUL bytes.

Add unit tests to test3211.

Closes #22628

projects/OS400/ccsidcurl.c

@@ -1205,7 +1205,7 @@ CURLcode curl_easy_setopt_ccsid(CURL *curl, CURLoption tag, ...)
     }
 
     result = curl_easy_setopt(curl, CURLOPT_POSTFIELDS, s);
-    data->set.str[STRING_COPYPOSTFIELDS] = s; /* Give to library. */
+    data->set.str_copypostfields = s; /* Give to library. */
     break;
 
   default: