libbb: auto_string() for efficient handling of temporary malloced stirngs

libbb: auto_string() for efficient handling of temporary malloced stirngs

Use it in libiproute: get rid of one static string buffer.

function                                             old     new   delta
auto_string                                            -      51     +51
ll_index_to_name                                      10      49     +39
buffer_fill_and_print                                169     178      +9
scan_recursive                                       378     380      +2
decode_one_format                                    732     734      +2
cmdputs                                              334     332      -2
static.cur_saved                                       4       1      -3
static.nbuf                                           16       -     -16
printable_string                                      94      57     -37
ll_idx_n2a                                            53       -     -53
------------------------------------------------------------------------------
(add/remove: 2/2 grow/shrink: 4/3 up/down: 103/-111)           Total: -8 bytes
   text	   data	    bss	    dec	    hex	filename
 939880	    992	  17496	 958368	  e9fa0	busybox_old
 939880	    992	  17480	 958352	  e9f90	busybox_unstripped

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