Rework of copy mode commands ("send-keys -X") to parse the arguments so

Rework of copy mode commands ("send-keys -X") to parse the arguments so
that flags may be detected propertly rather than just looking for
strings ("-O" and so on). Also add -C and -P flags to the copy commands:
-C prevents the commands from sending the text to the clipboard and -P
prevents them from adding the text as a paste buffer.

Note some of the default key bindings change to add "--" and any similar
custom key bindings using "send-keys -X" may need a similar change.

GitHub issue 4153.