Run codespell on the entire codebase

This tool detects and fixes common English spelling mistakes, with
generally very few mistakes.

Here is the command I used to generate this commit.  There were a few
changes that had to be done manually, and of course adding the ignore
file:
```shell
codespell -I .codespell-ignore -x .codespell-ignore -w
```

I didn’t add it to the CI, but this would be a good place for it.

src/tools/pw-dot.c

@@ -496,7 +496,7 @@ static int draw_graph(struct data *d, const char *path)
 	/* open the file */
 	fp = fopen(path, "w");
 	if (fp == NULL) {
-		printf("open error: could not open %s for writting\n", path);
+		printf("open error: could not open %s for writing\n", path);
 		return -1;
 	}