[klibc] Move SEEK_ constants to a common header file

POSIX states that the SEEK_ constants should be defined by stdio.h,
unistd.h, and fcntl.h.  The easiest way to do that without having to
worry about duplicate definitions is to move them to their own header
file with header guards and #include it in the other files.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
4 files changed