shells: do not frocibly enable test, echo and kill _applets_,

just build relevant source and use xxx_main functions.
build system: add a special case when we have exactly one applet enabled
(makes "true", "false", "basename" REALLY tiny).
getopt32: do not use stdio.

function                                             old     new   delta
getopt32                                            1385    1412     +27
make_device                                         1187    1200     +13
basename_main                                        120     127      +7
tcpudpsvd_main                                      1922    1926      +4
testcmd                                                5       -      -5
echocmd                                                5       -      -5
fuser_main                                          1243    1231     -12
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 4/1 up/down: 51/-22)             Total: 29 bytes

libbb/Kbuild

@@ -101,6 +101,7 @@ lib-y += wfopen_input.o
 lib-y += xatonum.o
 lib-y += xconnect.o
 lib-y += xfuncs.o
+lib-y += xfunc_die.o
 lib-y += xgetcwd.o
 lib-y += xgethostbyname.o
 lib-y += xreadlink.o