basic/extract-word: allow escape character to be escaped

With EXTRACT_UNESCAPE_SEPARATORS, backslash is used to escape the separator.
But it wasn't possible to insert the backslash itself. Let's allow this and
add test.

src/basic/extract-word.c

@@ -102,8 +102,8 @@ int extract_first_word(const char **p, char **ret, const char *separators, Extra
                                         else
                                                 sz += utf8_encode_unichar(s + sz, u);
                                 } else if ((flags & EXTRACT_UNESCAPE_SEPARATORS) &&
-                                           strchr(separators, **p))
-                                        /* An escaped separator char */
+                                           (strchr(separators, **p) || **p == '\\'))
+                                        /* An escaped separator char or the escape char itself */
                                         s[sz++] = c;
                                 else if (flags & EXTRACT_CUNESCAPE_RELAX) {
                                         s[sz++] = '\\';