Added stat_first_parent() helper function.

mkdir_parents_chown(): If gid is set, make sure setgid-bit isn't copied from parent.

Whenever file's group changing fails, show the group origin in the error message. --HG-- branch : HEAD

Added mkdir_chown(). --HG-- branch : HEAD

Added mkdir_parents_chown(). --HG-- branch : HEAD

mkdir_parents() API was sometimes assumed to return EEXIST and sometimes not. Standardized it now so that the API does return EEXIST. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

More robust error handling for mbox. --HG-- branch : HEAD