pbulk: add strict mode

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

In case anyone is interested, here is a patch adding a 'strict_mode'
option to pbulk.

When this is set to 'yes', pbulk will just exit if there is an
unresolvable dependency in the resolve phase, like it did before my
recent commit (i.e. before pbulk-base-0.58/pbulk-0.74).

It survived the same tests I did for the previous change
(strict_mode=yes or unset or no; limited or full bulk for only
SUBDIR=ham).

I will happily commit this if anyone speaks up for this option.

Let me know!

Thanks,
 Thomas