libbb: make bb_common_bufsiz1 1 kbyte, add capability to use bss tail for it

libbb: make bb_common_bufsiz1 1 kbyte, add capability to use bss tail for it

The config item is FEATURE_USE_BSS_TAIL. When it is off (default):

function                                             old     new   delta
read_config                                          210     228     +18
doCommands                                          2279    2294     +15
ipneigh_list_or_flush                                763     772      +9
ipaddr_list_or_flush                                1256    1261      +5
display_process_list                                1301    1306      +5
conspy_main                                         1378    1383      +5
do_lzo_compress                                      352     355      +3
do_lzo_decompress                                    565     567      +2
push                                                  46      44      -2
inetd_main                                          2136    2134      -2
uevent_main                                          421     418      -3
addLines                                              97      92      -5
bb_common_bufsiz1                                   8193    1024   -7169
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 8/5 up/down: 62/-7181)        Total: -7119 bytes
   text	   data	    bss	    dec	    hex	filename
 829850	   4086	   9080	 843016	  cdd08	busybox_old
 829901	   4086	   1904	 835891	  cc133	busybox_unstripped

FEATURE_USE_BSS_TAIL=y:

read_config                                          210     228     +18
doCommands                                          2279    2294     +15
ipneigh_list_or_flush                                763     772      +9
ipaddr_list_or_flush                                1256    1261      +5
display_process_list                                1301    1306      +5
conspy_main                                         1378    1383      +5
do_lzo_compress                                      352     355      +3
do_lzo_decompress                                    565     567      +2
inetd_main                                          2136    2134      -2
bb_common_bufsiz1                                   8193       -   -8193
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 8/1 up/down: 62/-8195)        Total: -8133 bytes
   text	   data	    bss	    dec	    hex	filename
 829850	   4086	   9080	 843016	  cdd08	busybox_old
 829911	   4086	    880	 834877	  cbd3d	busybox_unstripped

FIXME: setup_common_bufsiz() calls are missing.

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