spelling: call it null-terminate consistently

With a dash, using two Ls. Also for different forms of the word.

Use NULL in all uppercase if it means a zero pointer.

Follow-up to 307b7543ea

Closes #17489

src/tool_cb_hdr.c

@@ -248,7 +248,7 @@ size_t tool_header_cb(char *ptr, size_t size, size_t nmemb, void *userdata)
       if(hdrcbdata->honor_cd_filename &&
          hdrcbdata->config->show_headers) {
         /* still awaiting the Content-Disposition header, store the header in
-           memory. Since it is not zero terminated, we need an extra dance. */
+           memory. Since it is not null-terminated, we need an extra dance. */
         char *clone = aprintf("%.*s", (int)cb, str);
         if(clone) {
           struct curl_slist *old = hdrcbdata->headlist;
@@ -440,7 +440,7 @@ void write_linked_location(CURL *curl, const char *location, size_t loclen,
   if(!u)
     goto locout;
 
-  /* Create a NUL-terminated and whitespace-stripped copy of Location: */
+  /* Create a null-terminated and whitespace-stripped copy of Location: */
   copyloc = malloc(llen + 1);
   if(!copyloc)
     goto locout;

src/tool_getparam.c

@@ -78,7 +78,7 @@ static ParameterError getstrn(char **str, const char *val,
     return PARAM_NO_MEM;
 
   memcpy(*str, val, len);
-  (*str)[len] = 0; /* null terminate */
+  (*str)[len] = 0; /* null-terminate */
 
   return PARAM_OK;
 }

src/tool_parsecfg.c

@@ -237,10 +237,10 @@ int parseconfig(const char *filename, struct GlobalConfig *global)
 
 /*
  * Copies the string from line to the buffer at param, unquoting
- * backslash-quoted characters and NUL-terminating the output string.
- * Stops at the first non-backslash-quoted double quote character or the
- * end of the input string. param must be at least as long as the input
- * string. Returns the pointer after the last handled input character.
+ * backslash-quoted characters and null-terminating the output string. Stops
+ * at the first non-backslash-quoted double quote character or the end of the
+ * input string. param must be at least as long as the input string. Returns
+ * the pointer after the last handled input character.
  */
 static const char *unslashquote(const char *line, char *param)
 {
@@ -317,7 +317,7 @@ static bool get_line(FILE *input, struct dynbuf *buf, bool *error)
 }
 
 /*
- * Returns a line from the given file. Every line is NULL terminated (no
+ * Returns a line from the given file. Every line is null-terminated (no
  * newline). Skips #-commented and space/tabs-only lines automatically.
  */
 bool my_get_line(FILE *input, struct dynbuf *buf, bool *error)