Add a batchpipe() command to helpers that behaves somewhat like xargs(1).

Add a batchpipe() command to helpers that behaves somewhat like xargs(1).

Add batchpipe(), which will yield the output produced by calling a
given external command with a given list of arguments.

The resulting output may be provided in chunks, from multiple
invocations of the command, if the limits imposed by ARG_MAX make that
necessary.

See http://www.in-ulm.de/~mascheck/various/argmax/ for details, but
note that batchpipe() takes the additional precaution of adding room
for the argv pointers in addition to the envp pointers.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>