crontab: almost complete rewrite

crontab: almost complete rewrite
crontab: make options consistent with other implementations

   text    data     bss     dec     hex filename
   2042       4       0    2046     7fe busybox.t1/miscutils/crontab.o
   1331       0       0    1331     533 busybox.t2/miscutils/crontab.o

function                                             old     new   delta
edit_file                                            733     956    +223
open_as_user                                           -     171    +171
packed_usage                                       23652   23650      -2
CDir                                                   8       4      -4
ChangeUser                                           139       -    -139
crontab_main                                        1522     616    -906
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(add/remove: 1/1 grow/shrink: 1/3 up/down: 394/-1051)        Total: -657 bytes