su: move restricted_shell into su.c (the only user)

su: move restricted_shell into su.c (the only user)

function                                             old     new   delta
su_main                                              448     468     +20
buffer_fill_and_print                                179     196     +17
scriptreplay_main                                    205     208      +3
localcmd                                             275     277      +2
hash_find                                            233     234      +1
devmem_main                                          469     463      -6
install_main                                         724     716      -8
setusershell                                          11       -     -11
find_pair                                            187     169     -18
restricted_shell                                      49       -     -49
------------------------------------------------------------------------------
(add/remove: 0/3 grow/shrink: 5/3 up/down: 43/-92)            Total: -49 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>