xargs: simplify logic

Removed double-buffering in a linked list.

function                                             old     new   delta
store_param                                            -      56     +56
xargs_main                                           871     840     -31
process_stdin                                        458     396     -62
process0_stdin                                       267     143    -124
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(add/remove: 1/0 grow/shrink: 0/3 up/down: 56/-217)          Total: -161 bytes

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