ftpd: reuse ls applet for LIST/NLST/STAT generation

ftpd: reuse ls applet for LIST/NLST/STAT generation

function                                             old     new   delta
popen_ls                                               -     211    +211
ftpd_main                                           1760    1826     +66
handle_dir_common                                    199     228     +29
get_remote_transfer_fd                                89     104     +15
replace_char                                          30      34      +4
handle_upload_common                                 263     265      +2
bind_for_passive_mode                                129     121      -8
cmdio_write                                           84      62     -22
escape_text                                          166     136     -30
init_data_sock_params                                 81       -     -81
ftpdataio_dispose_transfer_fd                         87       -     -87
write_dirstats                                       149       -    -149
write_filestats                                      603       -    -603
------------------------------------------------------------------------------
(add/remove: 1/4 grow/shrink: 11/5 up/down: 384/-986)        Total: -602 bytes
   text    data     bss     dec     hex filename
 808804     476    7864  817144   c77f8 busybox_old
 808156     476    7864  816496   c7570 busybox_unstripped