btrfs-progs: send: check if parent or clone sources are read-only

The reference roots have to be read-only otherwise the results of send
stream could be undefined if the roots accidentaly change during the
process.

Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: update send help strings and manpage

- send accepts multiple subvolumes
- add missing option -e to man
- minor man formating fix

Signed-off-by: David Sterba <dsterba@suse.cz>