Update bup-split(1); document -d, clarify the split "modes", and reorganize.
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 <143a18b53deb6612d517b2e11f8086b561f1df57@lelutin.ca>
[1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@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 <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

gevaerts/bup,gevaerts/bup,ToxicFrog/bup,mhoeher/bup,ToxicFrog/bup,jbaber/bup,tjanez/bup,pombredanne/bup,gevaerts/bup,mhoeher/bup,tjanez/bup,mhoeher/bup,ToxicFrog/bup,tjanez/bup,pombredanne/bup,tjanez/bup,tjanez/bup,ToxicFrog/bup,gevaerts/bup,tjanez/bup,jbaber/bup,pombredanne/bup,mhoeher/bup,jbaber/bup,mhoeher/bup,pombredanne/bup,pombredanne/bup,gevaerts/bup,pombredanne/bup,ToxicFrog/bup,jbaber/bup,jbaber/bup,mhoeher/bup,jbaber/bup