add new linux file sealing api to fcntl.h

add new linux file sealing api to fcntl.h

new in linux v3.17 commit 40e041a2c858b3caefc757e26cb85bfceae5062b
sealing allows some operations to be blocked on a file which makes
file access safer when fds are shared between processes (only
supported for shared mem fds currently)

flags:
F_SEAL_SEAL prevents further sealing
F_SEAL_SHRINK prevents file from shrinking
F_SEAL_GROW prevents file from growing
F_SEAL_WRITE prevents writes

fcntl commands:
F_GET_SEALS get the current seal flags
F_ADD_SEALS add new seal flags