inetd: use change_identity().

inetd: use change_identity().
libbb: shrink our internal initgroups().
httpd: remove stray 'else' and 'index_page = "index.html"'

function                                             old     new   delta
httpd_main                                           750     743      -7
inetd_main                                          2033    2011     -22
bb_internal_initgroups                               251     228     -23
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(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-52)             Total: -52 bytes