build: fully omit verbose strings and code when disabled

When the compiler supports C99.

- map logging functions to macro stubs when verbose logging is disabled
  and the compiler is C99. Make sure these stubs silence unused variable
  warnings for non-variadic arguments.
  Before this patch they mapped to function stubs, the same codepath
  used for C89 compiler in this configuration.

- introduce new macros to tell the compiler which code to include
  when verbose code is active, or inactive:

  - `CURLVERBOSE`: defined when verbose code is active.
    To enclose blocks of code only used for verbose logging.

  - `VERBOSE(statement);`:
    compile statement when verbose code is active.
    To mark code lines only used for verbose logging.

  - `NOVERBOSE(statement);`:
    compile statement when verbose code is inactive.
    To suppress warnings for arguments passed to logging functions via
    printf masks, e.g. `NOVERBOSE((void)ipaddress);`, yet keeping
    the warning in verbose builds.

  Note these macros are not the same as `CURL_DISABLE_VERBOSE_STRINGS`.
  Verbose code is always active in C89 mode (without variadic macro
  support).

- drop existing uses of `CURL_DISABLE_VERBOSE_STRINGS` where redundant,
  or replace with the above macros. Ending up reducing the number of
  `#ifdef`s, and also the number of lines.

Assisted-by: Daniel Stenberg
Assisted-by: Jay Satiro
Reported-by: Dan Fandrich
Fixes #20341
Refs: #12105 #12167

Closes #20353

lib/vssh/libssh2.c

@@ -251,7 +251,7 @@ static LIBSSH2_FREE_FUNC(my_libssh2_free)
     Curl_cfree(ptr);
 }
 
-#ifndef CURL_DISABLE_VERBOSE_STRINGS
+#ifdef CURLVERBOSE
 static const char *myssh_statename(sshstate state)
 {
   static const char * const names[] = {
@@ -322,7 +322,7 @@ static const char *myssh_statename(sshstate state)
 }
 #else
 #define myssh_statename(x)    ""
-#endif /* !CURL_DISABLE_VERBOSE_STRINGS */
+#endif /* CURLVERBOSE */
 
 #define myssh_state(x, y, z) myssh_set_state(x, y, z)
 
@@ -334,7 +334,7 @@ static void myssh_set_state(struct Curl_easy *data,
                             struct ssh_conn *sshc,
                             sshstate nowstate)
 {
-#ifndef CURL_DISABLE_VERBOSE_STRINGS
+#ifdef CURLVERBOSE
   if(sshc->state != nowstate) {
     CURL_TRC_SSH(data, "[%s] -> [%s]",
                  myssh_statename(sshc->state),