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

tests/http/testenv/mod_curltest/mod_curltest.c

@@ -949,7 +949,7 @@ static int curltest_post_config(apr_pool_t *p, apr_pool_t *plog,
                                 apr_pool_t *ptemp, server_rec *s)
 {
   void *data = NULL;
-  const char *key = "mod_curltest_init_counter";
+  static const char *key = "mod_curltest_init_counter";
 
   (void)p;
   (void)plog;