Revert "Write the system ARG_MAX to an autogenerated config.py..."

Prepare to try a more direct approach.

This reverts commit b72365ad597edbc528da5b93528fc3a67a18d58e.
Read ARG_MAX directly via os.sysconf('SC_ARG_MAX').

For now, this code also adds an optional arg_max parameter to
batchpipe(), exclusively for testing.

There appears to be an issue with wvtest at the moment that causes
assignments to something like helpers.arg_max to have no effect from
the importers perspective.

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