wall: access FILE under real user's credentials

While at it, move applet/config/kbuild bits into wall.c.
(This way, it's more visible that applet is suid'ed).

function                                             old     new   delta
wall_main                                             87     138     +51

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
bunzip2: fix off-by-one check

stage3-armv7a_hardfp-20130209.tar.bz2, 149189948 bytes long,
md5sum b29ce23312e14eb15a143377d4a38473, was failing to unpack.

It so happened that this file has a run which exactly fills
the 90k buffer. The check was "size >= bufsize", apparently
it has to be ">".

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