Correct check_private_dir's dir mode

This commit attempts to satisfy nickm's comment on check_private_dir() permissions:
https://trac.torproject.org/projects/tor/ticket/11291#comment:12
"""check_private_dir() ensures that the directory has bits 0700 if CPD_CHECK_MODE_ONLY is not set. Shouldn't it also ensure that the directory has bits 0050 if CPD_CHECK_MODE_ONLY is not set, and CPD_GROUP_READ is set?"""