- Split if_clone.c out of if.c, license in if.c is attached to if_clone.c

- Split if_clone.c out of if.c, license in if.c is attached to if_clone.c
- Split if_clone.h out of if.h and if_var.h, license of if_var.h is attached
  to if_clone.h
- Staticize some variables and function in if_clone.c
- if_clonereq is the only userland visible structure related to this commit;
  it is kept in if.h for now, so userland application won't be aware of this
  commit.  It will be moved to net/if_clone.h

No functional changes.

# if_clone.c is subjected to change to support clone creation with
# additional parameters.