Cygwin: drop internal O_NOSYMLINK and O_DIROPEN flags

Cygwin: drop internal O_NOSYMLINK and O_DIROPEN flags

Both flags are outdated and collide with official flags in
sys/_default_fcntl.h, which may result in weird misbehaviour
of file functions.

O_NOSYMLINK is not used anyway.

O_DIROPEN is used in fhandler_virtual and derived classes.
The collision with O_NOFOLLOW results in spurious EISDIR
errors when, e. g., reading files in the registry.
fhandler_base::open_fs uses O_DIROPEN in the call to
fhandler_base::open, but it's not used in this context
further down the road.

Drop both flags and create an alternative "diropen" bool
flag in fhandler_virtual.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>