Add --pause-metadata option

This option pauses downloads created as a result of metadata
download. There are 3 types of metadata downloads in aria2: (1)
downloading .torrent file. (2) downloading torrent metadata using
magnet link. (3) downloading metalink file.  These metadata downloads
will generate downloads using their metadata. This option pauses these
subsequent downloads.
Document --pause-metadata option in man page
Pause download only when RPC is enabled