Use fewer flags when opening directories

Use fewer flags when opening directories

This doesn’t change behavior; it is a refactoring for
compatibility with a future patch that will use Linux’s
openat2 syscall, which is pickier about flags.
* src/tar.c (decode_options): When searching directories,
do not use O_NOCTTY, O_NONBLOCK, and O_NOATIME.
openat2 rejects all three flags if O_PATH is used.
The first two flags are definitely irrelevant for directories,
and O_NOATIME probably doesn’t matter either.