ftpd: do not use nasty tricks for re-execing if we are on MMU machine.

ftpd: do not use nasty tricks for re-execing if we are on MMU machine.
On NOMMU, code is alomost the same, on MMU:

function                                             old     new   delta
handle_dir_common                                    223     390    +167
ftpd_main                                           2306    2231     -75
popen_ls                                             203       -    -203
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 1/1 up/down: 167/-278)         Total: -111 bytes