curl: make --silent work stand-alone

- renamed the struct field to 'silent' to match the cmdline option
- make --show-error toggle independently of --silent
- make --silent independent of ->noprogress as well

By doing this, the three options --silent, --no-progress-meter and
--show-error should work independently of each other and also work with
and without '--no-' prefix as documented.

Reported-by: u20221022 on github
Fixes #10535
Closes #10536

src/tool_main.c

@@ -155,7 +155,7 @@ static CURLcode main_init(struct GlobalConfig *config)
 #endif
 
   /* Initialise the global config */
-  config->showerror = -1;             /* Will show errors */
+  config->showerror = FALSE;          /* show errors when silent */
   config->errors = stderr;            /* Default errors to stderr */
   config->styled_output = TRUE;       /* enable detection */
   config->parallel_max = PARALLEL_DEFAULT;