fix(credentials): open secret files with O_NOFOLLOW|O_NONBLOCK; drop dup includes

secret_file_open() previously opened --password-file/--early-input with
plain O_RDONLY, so a symlinked path was followed before the owner/mode
fstat gate ran, and an empty/planted FIFO could block fgets forever.
Open with O_NOFOLLOW|O_NONBLOCK|O_CLOEXEC (mirroring the dummy-key
sidecar): ELOOP now fails closed, and a writer-less FIFO yields EOF/EAGAIN
instead of hanging. Clear O_NONBLOCK again for regular files, where it is
a no-op, so their stdio read path is unchanged.

file.c: drop the duplicate <fcntl.h>/<unistd.h> includes (kept the first
occurrences).

Tests: a symlinked password file is rejected, and a writer-less named
FIFO fails cleanly without hanging.

src/shared/file.c

@@ -25,8 +25,6 @@
 #include "utils.h"
 #include "protocol.h"
 #include "xattr.h"
-#include <fcntl.h>
-#include <unistd.h>
 
 /* Files larger than this are not loaded whole for transfer (the sender streams
  * them); a whole-file digest is computed from the path instead.  Kept in sync