Update bup-split(1); document -d, clarify the split "modes", and reorganize.

Update the synopsis, document the "-d DATE" option, and make a clearer
distinction between normal options and the options that select the
split "mode".

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
[rlb@defaultvalue.org: add dashes to seconds-since-epoch; adjust the
 MODES text a bit and move -b down; make a few other edits and adjust
 commit message.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Update bup split -h output (optspec) to distinguish between split "modes".

Split it into three lines to clearly show that some modes don't fare
well with others.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
[rlb@defaultvalue.org: adjust commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Mention --git-ids in bup split -h output sample commands.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>