basic: add string_contains_word()

This wraps the common pattern of using extract_first_word() in a loop
to look for a matching word.

src/basic/string-util.h

@@ -280,3 +280,4 @@ char* string_erase(char *x);
 
 int string_truncate_lines(const char *s, size_t n_lines, char **ret);
 int string_extract_line(const char *s, size_t i, char **ret);
+int string_contains_word(const char *string, const char *separators, const char *word);