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/vssh/libssh2.c

@@ -1768,7 +1768,7 @@ static CURLcode ssh_state_sftp_realpath(struct Curl_easy *data,
     return CURLE_FAILED_INIT;
 
   rc = libssh2_sftp_symlink_ex(sshc->sftp_session, ".",
-                               curlx_uztoui(strlen(".")),
+                               curlx_uztoui(CURL_CSTRLEN(".")),
                                sshp->readdir_filename, CURL_PATH_MAX,
                                LIBSSH2_SFTP_REALPATH);
   if(rc == LIBSSH2_ERROR_EAGAIN)