Fix problem with debug output from export-flavor garbling the gen_dep_list
result. The solution is different from the patch suggested in the PR, since
the debug message was only meant to be used during early development of
flavors support. In my local development version of portmaster, all output
from pm_v is redirected to STDERR, but that change seemed to disruptive to
apply to the "published" version.
Unrelated changes: make pm_isdir_pd more robust and fix variable name of
BACKUP_FORMAT option in sample file.
Reported by: John Hein
Approved by: antoine (implicit)