syscallcompat: Use O_PATH to open base directory.
syscallcompat: Use O_PATH to open base directory.

Also remove some unnecessary flags: When O_PATH is specified in flags, flag
bits other than O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs