Use TARGETS_REGULAR if the recursive flag is not set

[daniel-ayers]

The program currently stores all targets into TARGETS_RECURSIVE even
if the --recursive flag is not set. This causes the program to
incorrectly snapshot only the most-ancestral datasets in the list. For
example, the following invocation would only snapshot datasets a and
b:

$ zfs-snapshot-auto a a\child1 a\child2 b b\child1 b\child2

Instead, we should be storing each valid dataset in TARGETS_REGULAR
so that they can be backed up individually. This commit tests wether
the flag is set before deciding whih variable to update.

Fixes: zfsonlinux#83