xargs: add support for -I and -i. Closes 493

xargs: add support for -I and -i. Closes 493

function                                             old     new   delta
process_stdin_with_replace                             -     195    +195
xmalloc_substitute_string                              -     145    +145
xargs_main                                           808     884     +76
count_strstr                                           -      45     +45
packed_usage                                       29580   29571      -9
parse_params                                        1445    1416     -29
func_exec                                            285     138    -147
------------------------------------------------------------------------------
(add/remove: 4/0 grow/shrink: 1/3 up/down: 461/-185)          Total: 276 bytes
   text	   data	    bss	    dec	    hex	filename
 922156	    932	  17692	 940780	  e5aec	busybox_old
 922440	    932	  17692	 941064	  e5c08	busybox_unstripped

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