libpwdgrp: store getXXnam result in a single malloc block

libpwdgrp: store getXXnam result in a single malloc block

This saves a bit of memory but more importantly, allows to create
xmalloc_getpwnam() API where result can be deleted simply using free().

function                                             old     new   delta
getXXnam                                             134     173     +39
parse_common                                         188     212     +24
convert_to_struct                                    277     290     +13
get_S                                                 90      88      -2
tokenize                                             129     126      -3
bb_internal_getpwent_r                               175     172      -3
getXXnam_r                                           208     198     -10
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(add/remove: 0/0 grow/shrink: 3/4 up/down: 76/-18)             Total: 58 bytes

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