sulogin: use common password-checking routine.

This needed some extensions correct_passwd() function,
which got renamed ask_and_check_password() to better describe what it does.

function                                             old     new   delta
ask_and_check_password_extended                        -     215    +215
ask_and_check_password                                 -      12     +12
vlock_main                                           394     397      +3
sulogin_main                                         494     326    -168
correct_password                                     207       -    -207
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 1/1 up/down: 230/-375)         Total: -145 bytes

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

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
libbb: move nuke_str() from passwd into libbb

function                                             old     new   delta
nuke_str                                               -      15     +15
ask_and_check_password_extended                      215     206      -9
init_main                                            781     771     -10
nuke_str                                              27       -     -27
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 0/2 up/down: 15/-46)            Total: -31 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
tar: fix exitcode check for MMU-spawned unpacking helpers

Testcase: tar xvzf EMPTY_FILE

function                                             old     new   delta
open_transformer                                     102     106      +4
get_header_tar                                      1781    1785      +4

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
tar: add a testcase for previous commit

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
fix a copy/paste error in last tar testcase name

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
tar: prevent empty file to be treated as valid tarball

function                                             old     new   delta
tar_main                                             879     895     +16

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
rfkill: use new-style config/kbuild/applet

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
awk: fix a bug in argc counting in recent change

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