[PATCH] doas: Make backslash mark words as non-keyword

Per doas.conf(5)
  If quotes or backslashes are used in a word, it is not considered a keyword.

Found with LLM. Patched and confirmed fix manually.

E.g. this is a parse error before the patch, but works after:

```
permit \keepenv
```

diff --git a/usr.bin/doas/parse.y b/usr.bin/doas/parse.y
index 604becb54..32f74f2ab 100644
--- a/usr.bin/doas/parse.y
+++ b/usr.bin/doas/parse.y
@@ -267,6 +267,7 @@ repeat:
                        escape = 0;
                        continue;
                case '\\':
+                       nonkw = 1;
                        escape = !escape;
                        if (escape)
                                continue;