Fix typo.

Rename nAtOne to nAtOnce.
Add nJobs check.

If HasMP is false, then prevent nJobs from being set higher than 1, even
if the user passes the -j flag.
Fix bug when only one input file is given.

Ahadd now runs a simple copy operation if only one input file is given.
Previously it threw a "Not enough arguments" error, but this is often
undesirable if ahadd is run as part of a script where the number of
files it will have to deal with is unknown.