networking/interface.c: get rid of global data

networking/interface.c: get rid of global data

These were data/bss:

static.proc_read                                       1       -      -1
int_list                                               4       -      -4
int_last                                               4       -      -4

We never call display_interfaces() twice, thus code to not scan /proc twice
never triggers.

function                                             old     new   delta
do_if_print                                            -    1998   +1998
display_interfaces                                   145     249    +104
static.proc_read                                       1       -      -1
add_interface                                        104     103      -1
int_list                                               4       -      -4
int_last                                               4       -      -4
if_readlist_proc                                     560     542     -18
if_readconf                                          141       -    -141
do_if_fetch                                          643       -    -643
ife_print                                           1296       -   -1296
------------------------------------------------------------------------------
(add/remove: 1/6 grow/shrink: 1/2 up/down: 2102/-2108)         Total: -6 bytes
   text	   data	    bss	    dec	    hex	filename
 933084	    473	   6844	 940401	  e5971	busybox_old
 933087	    473	   6836	 940396	  e596c	busybox_unstripped

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