kernel: Add three new commands to fctnl

kernel: Add three new commands to fctnl

This commit adds the following new commands to fcntl():
  F_DUP2FD         - non-portable functional equivalent of dup2(fd, arg)
  F_DUPFD_CLOEXEC  - A version of F_DUPFD that sets the close-on-exec
                     on the new file descriptor
  F_DUP2FD_CLOEXEC - A version of F_DUP2FD that sets the close-on-exec
                     on the new file descriptor.  It is non-portable

It also adds a missing break in a case statement for F_GETOWN in
sys_fnctl(), spotted by dillon.

reviewed-by: dillon