POSIX allows <fcntl.h> to include <sys/stat.h>, but doesn't require it.
There's little reason to do that, since they are separate headers, and
<fcntl.h> just needs the mode_t constants. Fix the code accidentally
relying on <fcntl.h> including <sys/stat.h>. The mode_t constants are now
provided in their own kernel header <sortix/mode.h>.

Additionally fix <sys/stat.h> pulling in all of <sys/types.h>, which is not
allowed by POSIX, which only requires a few types to be declared. Fix the
code accidentally relying on <sys/stat.h> including <sys/types.h>.

Finally fix <dirent.h> pulling in <stdint.h> through <sortix/dirent.h>.

The <sortix/__/dt.h> and <sortix/__/stat.h> headers are no longer required
and their contents have been merged into <sortix/__/dirent.h>.