Open a directory with the usual flags

Open a directory with the usual flags

Use O_RDONLY since you are not supposed to write to a directory.

Use O_DIRECTORY as mandated by POSIX (The Open Group Base Specifications
Issue 7, 2018 edition IEEE Std 1003.1-2017):

"If the type DIR is implemented using a file descriptor, the descriptor
shall be obtained as if the O_DIRECTORY flag was passed to open()."

Use O_CLOEXEC as mandated by POSIX:

"When a file descriptor is used to implement the directory stream, it
behaves as if the FD_CLOEXEC had been set for the file descriptor."

Drop the fcntl() call in favour of O_CLOEXEC.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>