tidy-up: use more static, sizeof(), char[], double-const

- make `const` data `static`, where missing and possible.
- replace `strlen()` on literal or const strings with `sizeof()`.
  While the latter is optimized by popular C compiler, e.g. MSVC only
  does it with `/O2`.
- replace magic numbers with `sizeof()`, where missing.
- introduce `CURL_CSTRLEN()` macro for `sizeof(char[]) - 1`.
- use `CURL_CSTRLEN()` macro.
- move `const` before integer types, where missing.
- replace `char *var` with `var[]`, where missing and possible.
- use double const, where missing.
  `static const char *` -> `static const char * const`.
- lib1514: constify pointers.
- unit3205: drop redundant cast, avoid another one.
- unit1666: map `OID()` macro to identical `STRCONST()`.

Closes #22406

lib/http_digest.c

@@ -54,7 +54,7 @@ CURLcode Curl_input_digest(struct Curl_easy *data,
   if(!checkprefix("Digest", header) || !ISBLANK(header[6]))
     return CURLE_AUTH_ERROR;
 
-  header += strlen("Digest");
+  header += CURL_CSTRLEN("Digest");
   curlx_str_passblanks(&header);
 
   return Curl_auth_decode_digest_http_message(header, digest);