Do not mutate default arguments

In this case, it looks rather harmless (since the conditions for --restrict-filenames should not change while a process is running), but just to be sure.
This also simplifies the interface for callers, who can just pass in the idiomatic None for "I don't care, whatever is the default".
Clarify --download-archive help
Clarify help of --write-pages (#1853)
Merge branch 'master' of github.com:rg3/youtube-dl