xargs: support -a FILE

xargs: support -a FILE

The GNU-specific option -a lets xargs read the arguments from a file
rather than from stdin.

This is particularly convenient when debugging in gdb interactively,
and it might be of more general use.

function                                             old     new   delta
xargs_main                                           788     823     +35
packed_usage                                       31683   31671     -12

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>