Fix default enum values in help output

nenum() does not return an enum value, but an index into the underlying
array, which means that the first element is 1 and not zero like the
code assumed.  The help output was off-by-one, and could error out if
some environment variable was set that was affecting the help output.

Fixes #1438.

Change-Id: Iec2f05c9ced55868902069ce59bc4979a6879f86