libpwdgrp: use getpwent() instead of getpwent_r()

libpwdgrp: use getpwent() instead of getpwent_r()

function                                             old     new   delta
massage_data_for_non_r_func                            -      90     +90
bb_internal_getpwent                                   -      69     +69
getXXnam_r                                            94     162     +68
fill_bounds                                          131     128      -3
deluser_main                                         355     310     -45
complete_username                                    123      78     -45
getXXnam                                             163      90     -73
massage_data_for_r_func                              103       -    -103
bb_internal_getpwent_r                               121       -    -121
------------------------------------------------------------------------------
(add/remove: 2/2 grow/shrink: 1/5 up/down: 227/-407)         Total: -163 bytes

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