Drop the ability to have a list of keys in the prefix in favour of two
separate options, prefix and prefix2. This simplifies the code and gets
rid the data options type which was only used for this one option.
Also add a -2 flag to send-prefix to send the secondary prefix key,
fixing a cause of minor irritation.
People who want three prefix keys are out of luck :-).
Permit multiple prefix keys to be defined, separated by commas, for example:
set -g prefix ^a,^b
Any key in the list acts as the prefix. The send-prefix command always sends
the first key in the list.
